Both sentences are correct but have a slight difference, which is difficult to explain...
When you just say either of these sentences, "は" sounds to me far more natural.
When you add "僕は" at the top (僕は値段が高いレストランは/があまり好きじゃない。）,
"が" is a bit more natural, while "は" sounds like focusing on "値段が高いレストラン"
implying that you like "値段の安いレストラン".
The same thing can be applied to similar sentences, for example,
"ピーマンは/があまり好きじゃない。", "猫は/があまり好きじゃない。" etc.
