What is this sentence using ～たら～たで trying to say about televisions and computers?

You already seem to understand the expression quite well so it is difficult to add to what you have said but the expression can also be used to say something is "nice to have (but not critical)". FWIW, I think the literal meaning of あったらあったで is:

"If we have it, we have it (and if we don't, we don't) but anyway/either way....."

Perhaps, rather than analyse the logic of the grammar, it might be easier to accept it as a set pattern and listen to when it is used. With luck after a while you will recognise it from the situation (my experience anyway).

As for your example, without knowing more about the context, I would say it means something along the lines of:

"Its nice to have modern amenities like PCs and TVs but we can get by without them."

Or, if you want a more literal translation:

"If we did not have modern amenities like PCs or TVs, well, we wouldn't have them. But we'd still get by, somehow."

If your textbook does not give any more context then that is probably because the writer thought it is easy to imagine somebody saying it as a comment on the modern world and the meaning is considered clear from なんとかなるものだ.

I guess you have other examples but here are couple from my old book (日本語総まとめ）which gives translations:

庭があったらあったで、草むしりが大変だ。 (=あるのはいいが、その場合は)
It is nice to have a garden but someone has to do the weeding.

便せんがなかったらないで、コピー用紙でもかまいません。(=あったらいいが、ない場合は)
If there is no letter writing paper available, photocopy paper will do.

□□したらしたで、＋ ○○しなさい

○○ is the appropriate action under the situation □□. If □□ is true, the listener should do ○○. ○○ can be anything expressing request or suggestions.

Example 2,3 are from Greek Fellows's answer.

1.1. したならしたで、正直に言いなさい
if you have done it(=□□) admit it(=○○) 

1.2. 電話に出(=□)たら出たで、礼儀正しく話し(=○)なさい
if you have to answer the phone(=□□), speak politely(=○○) 

1.3. 食べ終わったなら終わったで、食器を下げなさい。
if you have finished the meal(=□□), put away the dishes(=○○) 

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□□したらしたで + ○○

It doesn't matter even if □□した. ○○ is the reason or conclusion.

2.1. なかったらなかったで、何とかなるものさ
it doesn't matter if we don't have it (=□□),
because we can live without it (=○○)

2.2. 便せんがなかったらないで、コピー用紙でもかまいません。 (from Tim's post)
it doesn't matter if we don't have letter writing paper (=□□),
because photocopy paper will do (=○○)

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□□したらしたで + また○○

If □□ is not true, it is ●●. If □□ is true then it will become ○○.

●● is a not-so-good situation. ○○ is another one. It's a little contrastive.

Tim's example

3.1. 庭があったらあったで、草むしりが大変だ。
Not having a garden is not good (=●●).
But if we have a garden (=□□), we will have to weed it. (=○○)

3.2 若者が主張したらしたで「生意気だ」「甘えるな」と言われ、
    主張しなかったらしなかったで「内向き」「覇気がない」と言われる。

3.3 大学に入ったら入ったで、お金がかかって大変。

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□□したらしたで + ○○

□□ is bad, but but it can't be helped. ○○ is a kind of compromise.

4.1. 負けたら負けたで、次で取り返そう

4.2. 断られたら断られたで、仕方ない

4.3. 病気になったらなったで、喜んで薬を飲む

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Sometimes, I feel it □□したらしたで○○ simply means □□ has a corresponding result. ○○ is the result.

Sentences from 日本語表現文型辞典

5.1. プロになったらなったで、厳しい競争があるものさ。

5.2. 「話によると、あの子、名門中学に合格したそうよ」
     「あんな秀才ぞろいの受験校だと、入ったら入ったで、苦労すると思うよ。」

「電話に出たら出たで」 is hard to explain... maybe you might understand if I give you some examples:

• 「電話に出たら出たで礼儀正しく話しなさい。」 If you did answer the phone, then speak politely.
• 「食べ終わったなら終わったで食器を下げなさい。」 If you did finish the meal, put away the dishes.

In this case,

テレビでもパソコンでも、なかったらなかったでなんとかなるものだよ。

means:

Be it a television or a computer, you'll live fine without them.

According to this link here, 「やったらやったで」 can be swapped into 「やったとしたら、やったということで」.