Have you come across the concept of a relative clause yet? Look this up.
In English nouns are modified by adjectives. In Japanese nouns can also be modified by entire clauses. In this case 値段が高い is a sentence/clause in its own right with the meaning "the price is high". This is the relative clause here and it is used to modify the noun レストラン. So 値段が高いレストラン means "a restaurant where the price is high".
If I used たかいねだん レストラン, would it be wrong?
Yes it would be wrong because たかいねだん is a noun phrase and レストラン is a noun. You cannot put two nouns/noun phrases next to each other. But you know that you can modify a noun with another noun using の, right? So たかいねだんのレストラン would be grammatical. I'm not a native speaker so I'm not sure how natural that would be though.
The が particle indicates what is high/tall in the restaurant, right? This will always be indicated with a が particle?
There is a rule that (most of the time) in a relative clause you can only mark the subject by が (or の) and not by は. In relative clauses が can be replaced by の.
If I take away the ねだんが will the phrase then become something like: "I don't like tall restaurant very much"?
It could do but that would be an unusual thing to say. 高いレストランはあまり好きじゃない without any further context is ambiguous. It could mean that you don't like restaurants where the food is expensive, you don't like restaurants that you can't afford to buy, or, as you said, you don't like restaurants which are tall. Context is key in Japanese.