Both the original sentence and your version are correct sentences, but the meanings are different. To try to see the difference, try reading each sentence starting with the word before the は.

Original: レストランはあまり好きじゃない。- I don’t like restaurants.
If we put the rest of the sentence back:
[値段が高い]レストランはあまり好きじゃない。 – I don’t like [high priced] restaurants.

Yours: 値段はあまり好きじゃない。- I don’t like prices.
If we put the rest of the sentence back:
[レストランの高い]値段はあまり好きじゃない。 – I don’t like [restaurants' high] prices.

I think your trouble with the が might have something to do with the way nouns are modified in Japanese. I can’t speak for Spanish, but in English we modify nouns in two different ways. One way is by putting adjectives in front of the noun: “the red car”, “the big house”, etc. The other way is by putting long phrases after the noun: “the car my friend bought”, “the house down the street”.

In Japanese, anything used to describe a noun (no matter how long it is) comes before the noun. So you would say: “the red car” and “the big house”, but also “the my friend bought car” and “the down the street house”. Sometimes, these descriptive parts can get very long.

So to say “I don’t like restaurants with high prices.” you need to first say “I don’t like restaurants” (レストランは好きじゃない), and then add the part to describe the restaurant.

It might help to look at these three sentences, all of which have the same meaning:
値段が高いレストランはあまり好きじゃあない。
高い値段があるレストランはあまり好きじゃない
高い値段のレストランはあまり好きじゃない。

For the most part, yeah. The い is part of the noun-modifying phrase – it’s 値段が高い + レストラン.

But yeah, noun-modifying phrases can even be complete sentences, so it can be tricky spotting them sometimes. Main thing to remember is that they’ll never contain は.