Translation:These fish are very expensive.
Fish is one of those strange words in English where you don't change the word whether there's 1 of them or 100.
"Fish" can be singular or plural.
"Fishes" means, specifically, more than one type of fish. It is also unusual in everyday speech.
E.g. "There are many fish in that school." (There are many of the same kind of fish)
"There are many fishes in that aquarium." (There are many kinds of fish on display)
Joy to the world, To all the boys and girls Joy to the FISHES in the deep blue sea, Joy to you and me!
Could 非常 be translated as extraordinarily? I saw that 非 can be a morpheme equivalent to "not" in some words and 常 could be common. So 非常 = not commom (not ordinary) or extraordinary.