Idiom for something cheap that is expensive to replace

I would say "you got your original glasses for a bargain"

Bargain [bahr-guh n] /noun

1. an advantageous purchase, especially one acquired at less than the usual cost.

Source: Dictionary.com The sale offered bargains galore.

a sweet deal

a business or financial deal in which you get an advantage, pay a  low price etc

Usage: I got a sweet deal on the car.

source: Longman Dictionary of Contemporary English

You could as well use steal.

From Dictionary.com:

14. Informal. something acquired at a cost far below its real value; bargain:

This dress is a steal at $40.

I've looked for such a word (or short phrase) in the past and not found one. That probably means there isn't one in typical British English.

There are some terms which might help you though:

• "Replacement cost"/"replacement value" as in "it might have been cheap but it has a high replacement value"
• "Worth a lot to me". Note that this includes sentimental value.

The OP was asking for an idiom which

describes the current state of the sunglasses (from the example), not one that describes the initial purchase.

So it's not so much that it was cheap to purchase, but that it would be relatively expensive to replace.

To me, that speaks of an item which was once cheap and ubiquitous, but is now rare and hard to find, commanding a premium due to scarcity. A few examples:

• A popular soda brand alters its flavor; disappointed consumers pay a premium for the remaining stock of the old formula
• A legal product becomes illegal, for example a slightly-too-dangerous toy
• The newer versions of a product are cheaper and flimsier than the old ones

In such a case I might say

• They don't make them like they used to.
• This (pair of sunglasses) is an oldie but goodie.

A decorative object or toy which was produced for a limited time may become a collector's item, in some cases prized more for its rarity than its intrinsic worth.

Further from the example of the sunglasses, media, such as music, books, and especially movies which were unappreciated at the time of release may become a cult classic