Here is a phrase:
ねだんが たかいレストランは あまり すきじゃない
I don't like high price restaurant very much
Now, the questions.
Disclaimer: I'm still learning Kanji, so if possible, please add kana.
Your sentence:
値段{ねだん}が高{たか}いレストランはあまり好{す}きじゃない。
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.
Yes, it would be grammatically incorrect. たかい[値段]{ねだん} の レストラン would be the grammatically correct equivalent (notice the の particle).
First look at [値段]{ねだん}がたかい separately : it means "The price(s) is/are high". が marks the subject of a sentence in Japanese. By the way yes, [値段]{ねだん}がたかい is a sentence, even though you may notice there isn't any verb per se : a Japanese sentence doesn't need a verb like an English one does (furthermore, Japanese adjectives can be considered some kind of a verb).
The structure encountered here is the one of a relative clause, similar to the ones built with which for example in English. Consider the English literal translation :
I don't like restaurants of which the prices are high.
It's made up of 2 sentences : I don't like restaurants and The prices are high, linked semantically together through of which (or more naturally where).
The Japanese sentence is similar, it's made up of レストランはあまり[好き]{すき}じゃない and [値段]{ねだん}がたかい (direct literal translation of the 2 above English clauses).
Now, in Japanese, these 2 clauses are linked by putting the 2nd one in front of the noun it qualifies (here, レストラン).
So : [値段]{ねだん}がたかい → The prices are high ; [値段]{ねだん}がたかいレストラン → Restaurants of which / where the prices are high.
Note that in the relative clause, subject-marking が particle is sometimes replaced by the の particle. In this example : [値段]{ねだん}のたかいレストランはあまり[好き]{すき}じゃない would be a complete equivalent, with the same meaning.
Also note that, if the relative clause ends with a noun or a na-adjective for example, there could be an additional particle (for example, [周り]{まわり}が[静か]{しずか}なレストランは[好き]{すき} / I like restaurants of which the surroundings are calm -- notice the な between the 2 clauses), but the construction is the same : the relative clause is put directly in front of its noun antecedent.
I suggest you check the wikipedia explanation, it does a good job at showing how relative clauses are, in a way, "adjectified" sentences in Japanese.
Yes and no. Grammatically, [高い]{たかい}レストランはあまり[好き]{すき}じゃない is correct, and could mean both "I don't like expensive restaurants" or "I don't like high / tall restaurants".
It will mostly depend on context (e.g. the discussion it fits in). Note that, though, "high restaurants" is quite an unnatural expression and I think not very clear of what is meant, and that would be the same in Japanese. If "a restaurant situated high above the ground / high in a building / in a tall building" is meant, just as it would be mentioned more specifically in English, it would also be in Japanese (unless the context has already explicitly established what is meant by たかい). So in this particular example, and without further context, たかいレストラン would most likely be interpreted as "an expensive restaurant".
[値段]{ねだん}がたかいレストラン has the advantage of being more specific, and of teaching you of relative clauses in Japanese ;)
