Well, any fish including the price of it's food and set-up will hit at least fifty and go up. I have dropped at least $500 on my 55g. My bettas are around $40 a piece, but Marlboro (my favorite betta) has probably cost me like $150? I'm going to be getting him another new tank, one much nicer. So... into the $200's.
Buying just a fish? I would easily spend over a hundred if it was something special and I was all ready for it. I want a gar (probably a spotted or Florida) when I get out of college and settled, so I will be dropping a lot of cash.