The wealth of a farmer is noteworthy

Wealth is relative to your point of view. While we can look at Tatooine and see it as a poor locale compared to the rest of the civilized galaxy, on the local level there are still those who would be considered rich and poor. While Luke was a simple farm boy, and probably not a rich one, there isn't an indication that he was a dirt poor one either.

But their life is their moisture farm, which means much of what they do have goes towards maintaining it. If they don't have the tools and equipment they need to do the job, then they lose money in the long run, and more/better tools and equipment means more money. Consider how much tractors cost in the real world, and you might reconsider the idea of farmers being unable to afford what they need to do their jobs.

The value of a droid is contextual

With that in mind, lets look at the situation. The Jawas make their living selling captured droids. Considering their desert trawling nature, it would be reasonable to assume that moisture farmers make up a good portion of their clients. This means they need to price their wares in a way that is worthwhile to them, while still affordable to the farmers. Regardless of the "galactic" value of any given droid, no one is going to buy it if it is beyond what they can afford. This means that even some relatively clean and high quality droids like R2 and C-3PO are going to be sold at "used car" value, at prices fitting the local economy.

Consider everything R2 is capable of and his origins. Serving aboard a queen's vessel, a valuable droid throughout the entirety of the Clone Wars, and then in service on a "diplomatic" vessel. Clearly on the galactic scale, he's seen as a valuable astromech. While C-3PO's record isn't as spectacular, there's no reason (especially with his gold plating and translational prowess) to regard him as merely a piece of junk. All polished up on the galactic market they'd no doubt fetch a pretty penny.

The gift to Jabba is simply a token (but not really)

A peasant giving their last chickens to a king as tribute isn't insignificant. Wealth is relative, and Jabba is certainly aware of his own and the lack of others. While the droids are of more value at the scale that Jabba operates, it really isn't about their value to him; it's about their value from the one who gives, and the significance that tribute holds. He knows Luke isn't some rich dude trying to get a multi-million dollar favor. He's a poor man trying to make a deal for his friends by giving all he has of any value, as a tribute to Jabba's "greatness".

But of course, that's really just a setup. The real purpose was that Luke needed the droids there for his plan to work, and this was the most surefire way of accomplishing that. Luke knew Jabba would see little to no value in appeasing Luke, so there was no way the tribute would have swayed him anyways. Jabba has no reason to send them back, and not suspecting that Luke was capable of actually doing him any harm, he decides to mock him by accepting them anyways. His palace is certainly big enough for him to find something for them to do (which he does), and thus the trap has been set.

Luke doesn't say that they're "valuable", he vouchsafes that they're hardworking, and that they're a token of how serious he is to begin negotiations for Han's life.

I seek an audience with Your Greatness, to bargain for his life.

Note that the droids aren't the cost of Han's release, just the price of gaining an audience with "Mighty Jabba" to discuss the terms of his release.

“As a token of my goodwill, I present to you a gift: these two droids. Both are hardworking and will serve you well.”

For any employer that has a reputation for going through droids at an alarming rate, the presentation of two stable workers must be at least worthy of an eyebrow raise. Since Jabba has no interest in even meeting with Luke, he considers it a gift with no strings attached.

In addition to what others have said, I'd also like to point out that there is a difference between personal wealth and business capital. While Uncle Owen may not make enough on his farm to have frivolous disposable income, that doesn't mean the farm as a business is bankrupt.

Take a look at these tractor prices. They range from $300,000 to $500,000, which makes them comparable to luxury and sports cars in price tag, yet farmers who own them can't afford a car like that. The business has capital and it buys things because they are useful. But a sports car isn't useful for farming. It may be that the droids are quite valuable.

Droids could be of low value to moisture farmer, but very valuable to powerful crime lord.
Imagine having luxury sports car in impoverished country with only dirt or otherwise poor roads. Now imagine that car being "acquired" and sold by shady junk dealers with reputation of "loose" morality and penchant for swindling and outright theft. Obviously, price would be much lower than on open world market. Similarly, Jawas didn't have access to galaxy wide market, I doubt they could even sell freely in Mos Eisley, because there could be Imperial presence there or some other form of law enforcement.

Could Jawas sell directly to Jabba?
Probably not, as mentioned Jawas were small time crooks, Jabba's men would most likely either take whatever they like from them, or at least pay very meager prices. I suspect Jawas were paying protection racket to Jabba to be even able to operate on Tatooine. It would be wise from them to stay clear from his palace.

What is real worth of C-3PO?
C-3PO is a protocol and translation droid, "fluent in over seven million forms of communication". He was in diplomatic service, with Padme Amidala and the Organas. He would not be of much worth to a moisture farmer; Owen Lars intended to use him to operate moisture vaporators. On the other hand, Jabba did have a court, he has dealings across the Galaxy. A translation and etiquette droid could be very useful for him, especially since he could nor rely on brute force alone in the Age of the Empire.

What is real worth of R2-D2 ?
Astromech droid, not really of much worth to moisture farmer. In fact, Owen bought him only after another unit malfunctioned, probably to use him as a standard repair and mechanic droid, waste of his true abilities. On the other hand, Jabba is in smuggling business, his vessels need to travel across the galaxy, often taking the shortcuts. One of primary function of astromech droids is to calculate hyperspace jumps, very useful if you need to avoid scrutiny from law enforcement or potential rivals.

Therefore, with all this things in mind, Luke's gift is not at all worthless, especially if it is only a token of goodwill, something to start negotiations about a real price for release of Han Solo.

It is plausible that the Jawas didn't know the true worth of C-3PO or R2-D2.

Jawas are desert-dwelling scavengers who are just out to make money quickly, and will resort to theft to do so. As such, they can offer goods at lower prices and still make a profit. To the Jawas, a fully functioning droid they found in the desert is nothing but a means of earning money. They want a fast sale in case the real owners come looking for the droids and any money they make is a profit so they can afford to offer them for a knockdown price. Similarly criminals in the real world will sell expensive stolen technological equipment (e.g. phones, cameras) for much less in order to make a quick sale and avoid questions about the device's origins.

Jabba on the other hand is more intelligent and more well informed and would be able to recognise a protocol droid and an astromech droid and know what they are worth and more specifically what they are worth to him. A droid capable of acting as a translator and a droid capable of repairing any damage to his palace (and serving drinks) would both be of incredible use to Jabba. More to the point, from Jabba's point of view both were offered up to him as a gift in a supposed effort to make him more likely to agree to a deal, and in his arrogance he never considered for a moment that they might be part of an elaborate plot to free Solo.

So really neither of these incidents are a good indication of what the price of these droids are.

Their value on the other hand, like the value of any technological device, is in the eye of the beholder. To make a contrast with mobile phones, some people are willing to pay $200 for a phone because they are constantly using them and want the most powerful or trendiest phone available. To other people, $100 is the most they're willing to pay for because they don't use their phones as often and don't see the point in paying top-whack for one.

The same could be said of cars. Some people can afford Aston Martins, Rolls Royces and Ferarris and are prepared to pay for them, some people are happy with a 3-door Mini because to them a car is just transport.

I don't think that you can consider the value of these droids together. One is way more valuable than the other.

C-3PO is an example of what must be a fairly common piece of equipment in a galactic society. That particular unit, while it's been put to the service of a galactic senator and then on board a senate corvette, is recycled from parts readily scavenged by a slave boy. So it must be pretty low value.

R2-D2 however is clearly a prized model of astromech—note the astonished cries of Anakin's friend:

KITSTER : Wow, a real Astro Droid...how'd you get so lucky?
_{Star Wars I script}

Maybe Astromechs enjoy a "prestige" status? The Nubian ship model on which R2-D2 first served was in use by royalty and could be considered luxury/classic:

WATTO : Not bad...not bad...a Nubian.
_{Star Wars I script}

so I would think the droids that were outfitted for the ship would be in a similar esteem. The later model astromech BB-8 would fetch a relatively pretty sum for Rey in The Force Awakens:

UNKAR: What about the droid?

REY: What about him?

UNKAR: I'll pay for him.

BB-8 doesn't like this at all. Rey is awkward, but curious.

UNKAR (CONT'D): Sixty portions.
_{Star Wars VII script}

Of course, Unkar's portions seem to vary in value depending on his mood, so we can't really use this as a measuring stick.

As farm equipment R2-D2 and C-3PO might be worth less than a hired farm hand. Owen could afford to buy the droids but not to hire help despite Luke's wishes:

OWEN: Harvest is when I need you the most. Only one more season. This year we'll make enough on the harvest so I'll be able to hire some more hands. And then you can go to the Academy next year.
_{Star Wars I script}

But, when R2-D2 escapes, Luke knows there will be "hell to pay" if he's found out, and even risks going into the Jundland wastes to rescue it:

BEN: The Jundland wastes are not to be traveled lightly. Tell me young Luke, what brings you out this far?

LUKE: Oh, this little droid!
_{Star Wars I script}

So whatever Owen Lars paid for the droids, it wasn't a trivial amount. Note though, Luke almost forgot the protocol droid after the scrape with the Sand People:

BEN: I think we better get indoors. The Sandpeople are easily startled but they will soon be back and in greater numbers.

Luke sits up and rubs his head. Artoo lets out a pathetic beep causing Luke to remember something. He looks around.

LUKE: Threepio!
_{Star Wars I script}

---

On the other hand, as Luke's "token of goodwill", Jabba's servants don't recognise any special value in R2-D2 and put him to work serving drinks on a sail barge—perhaps it would make a good talking point and give bragging rights to business partners? But, they do recognise C-3PO as having more value to them:

NINEDENINE (to a Gamorrean guard)

Guard! This protocol droid might be useful. Fit him with a restraining bolt and take him back to His Excellency's main audience chamber.
_{Star Wars VI script}

I guess, even on Tatooine where droids are shunned, the value of a droid depends greatly upon your own point of view?

There is a lot of great evidence and reasoning in the other answers.

Let me also offer this hypothesis:

The droids actually are very valuable.

but

The Jawas actually don't know the value of the droids.

Why?

Jawas never see droids in this good conditions. They are junk dealers, not technicians and would never encounter such valuable droids. We know from the poor R2 unit with a bad motivator that their wares are not in good shape. And why would such valuable droids ever show up in the armpit of the quadrant to be scavenged?

Now, people are pointing out that the value of a good is contextual. That is true. But it also opens an arbitrage opportunity. If the value of these goods really do vary wildly between Tatooine and other locations, then there are traders who would snap up the goods and sell them elsewhere. We know there are lots of people at Mos Eisley who have the means and are in search of business opportunities.

Since the Jawas clearly are not charging much for C-3PO and R2-D2 droids, we must conclude that it almost never happens. That's the only way they could be naive enough about the value.

I think there is a false juxtaposition to this question.

The kind of droids that a desert farmer can expect to be able to acquire from junk-trading Jawas is probably usually of minimal value. The kind of droid a pack of junk-scavenging Jawas can expect to find, abandoned on a desolate nowhere planet like Tatooine is probably of minimal value.

However, these droids came to be where they were under rather extraordinary circumstances, which were unknown to the Jawas or anyone else, at first. This lack of knowledge about their origin, their less than stellar exterior condition from the circumstances of their escape and lack of maintenance available to them contributed to the assumptions/pre-judgements others had going in.

These droids were, in fact, of high enough quality that they were owned either personally or by the retinue of planetary royalty. That would indicate a valuable droid.

So, their basic value is probably pretty high. A Jedi knight, offering well-maintained droids worthy of being bought by royalty would not necessarily be questioned as being Jawa junk.

Plus, you know, quality and value is a relative assessment.

Adding to the question of value. Anakin did not sell C-3PO for his and his mother's freedom or food. The novel mentions they are close to the breadline with some quote about Qui-Gon seeing she was concerned about how to feed extra hungry mouths.

If the Jawas were selling to people who were unable to pay more then there is a good chance that they would sell the droids off world instead as there seems to be a reasonable transport network.

The droids Jawas sell are assumed to be good quality as Owen was angry when the first astromech purchased broke down. He didn't inspect the droid before and took the droid on faith.

The only explanation I can think of is that these droids are considered collectors items. R2-D2 particularly was heavily involved in the destruction of the Death Star.

Jabba values the droids like people, not equipment.

Jabba has a severe staffing problem, and Luke recommends the droids as good staff

Jabba is frustrated, professionally embarrassed, and at his wits end because he can't find competent staff with the right qualities (More on this later) Since individual droids have fundamental personal qualities (more on that in a bit), and Jabba's staff have demanding jobs, (and Jabba is picky and temperamental) he needs the right individuals, not just certain models of droids. He hasn't been able to find individual droids that suit him. The Droids excite Jabba because Luke testifies to their suitability, suggesting they might solve his problem:

"Both are hardworking, and will serve you well."

On Tatooine, the droids are valued only for their menial labor, and their personal qualities are ignored.

The Jawas could've taken the time to see that R2 is the feisty, quick-thinking lion-hearted hero he is and found a better buyer and a better price, but they just wanted to make the sale quickly. Owen doesn't interview them (even cuts off Threepio's elevator pitch) because most any individual can do menial farm-hand work, or can learn quickly.

Droids have individual qualities

Everyone on earth could be considered to be a 'Human MK1' model of creature. We are all the same species, the same equipment. Yet individual humans are of very different value to a master. When slaves were traded in earth history, they weren't all valued equally. Even among two men of equal physical strength, one with a reputation for hard work and years of experience would be valued much more than one with an insubordinate history and no applicable skills.

Droids of the same model have shown differences in ethics, temperament, personality, and mental proclivities. Furthermore, there's evidence that these traits cannot be arbitrarily reprogrammed. When R2 quibbles with Jabba's Chief of Cyborg Operations, EV-9D9, this line happens:

"You're a feisty little one, but you'll soon learn some respect"

Note that she doesn't say:

"Your personality has drifted out-of-spec and become feisty. I'll just run this Perl script to make you obedient."

A droid's temperament cannot be changed easily. Like a person, individual qualities of a droid make it more or less valuable to particular masters. And that line brings up another point:

Droids can learn

Which means that some droids are better suited to a job because of experience. Knowledge and skills aren't always enough. Real-world experience is important. Like a person, a droid is worth more than another of the same model if it has better experience.

So like a person (or slave) the suitability of a droid for a particular job depends on a droid's individual characteristics and career experience. The droids were of average value on Tatooine because no one realizes or cares about their individual potential. But for Jabba, who's trying to run a crime syndicate,

"A good droid is hard to find"

Jabba wants the droids to serve in his court, which is where his friends, business associates, and enemies (not mutually exclusive) come play politics and organized crime, sometimes with a thermal-detonator in-hand. Consider a King hiring a courtier or Al-Capone hiring an assistant. Even among the people with the right experience on their resume, it can be hard to find a good fit. Many personal characteristics—confidence, composure, temperament, demeanor, shrewdness, empathy (or lack of)—must be finely tuned to do well in a position like that. You need someone with enough experience to handle sensitive politics and work under pressure, but you also need someone who's personal qualities reflect the values (powerful, organized, ruthless) of you and your business when facing your space-mob frenemies.

Jabba's has a staffing problem both chronic and urgent

He hates most of his current and former staff. The last interpreter was disintegrated and there are many other droids being tortured in the dungeon presumably for pissing him off. The only droid that seems comfortable in the space-cartel trade is their torturer, and Jabba's Chief of Cyborg Operations, EV-9D9, who has the rare qualities of sharp authoritarianism, ruthless professionalism, and wry sadism. Most droids, like most people, probably don't have the right personality, ethics, temperament and guile to work in the court of an irritable hedonist space mobster. Although he can probably afford any model of droid many times over, he's having difficulty finding the right individuals.

Jabba's constant dysfunction with his attendants is a disaster

Jabba loses his composure and rages at his helpers in full view of the court. The seedy clients and spies see the dysfunction and circle like vultures. Jabba isn't even sure he can trust his repugnant majordomo, Bib Fortuna. Jabba is at his wits end trying and failing to find the right people, destroying droids and even killing dancers in exasperation and rage. Dysfunction makes him appear angry but inept. This is all a bad look for a crime syndicate and he probably knows it.

Luke seems to be offering the cure to Jabba's most enraging and embarrassing problem.

It's possible Luke knew about this situation. But even if it was a lucky guess, the droids seem like a perfectly timed salvation to Jabba's woes. This chronic problem has just come to crescendo now that he's disintegrated his last interpreter and has an urgent vacancy. Now C-3P0 and R2 don't actually have the right personalities either (3P0 is awkward and easily flummoxed, R2 is too feisty) but Jabba doesn't know that right away. Jabba is irritable, but also excitable. When Luke says:

"Both are hard working, and will serve you well"

He gets excited because Luke seems to be saying these droids have the right qualities and experience. It seems plausible because they have been serving a Rebellious politician and a powerful Jedi. They could be steely, shrewd, rebel-trained agents that will help him regain control of his court.

As the idiom goes, "It's impossible to find good help these days".