They both mean that one thing is expensive. Normally, we use '贵' in spoken Chinese，and we use ‘昂贵’ in writing Chinese. Of course， you can also use ‘贵’ in writing.

贵 you Can afford it.But it is not cheap 昂贵 you Can not afford It And It is valuable

两个词的意思是一样的。都是形容东西价格高。 “贵”在口语中最常用，“昂贵”在书面语中最常用。 “贵”形容绝大多数的东西，比如“香蕉多少钱一斤？8块一斤，有点贵” “昂贵”形容价钱很高的物品，比如“这条项链几万块钱，太昂贵了”

貴 = noble and by extension, expensive since only nobility can afford them. 花开富貴, 楊貴妃, 华貴, 矝貴 昂贵 = expensive.

贵is more expensive than 昂贵