In the $20 range. I might consider up to $70 for something I really want. There are many interesting fish that don't cost a lot of money.
It's not that I can't afford a more expensive fish. I just don't want the worry. I would go crazy if I had a $1,000 fish, worrying that it was going to die on me. I get more enjoyment and relaxation out of less expensive fish. Besides a gulper cat, I can't think of any expensive fish at the moment that would interest me.
The most I've spent so far was $20 for a blue crayfish(I know not a fish.) This was almost 10 years ago and I had to special order.
The most I've spent on a fish was a $8 for an Albino senegal bichir.