I paid 11.99 for a Bamboo Shrimp, but that's a shrimp, so it doesn't really count. For a fish, I paid 8 dollars for a BN Pleco so far, but I plan to buy a German Blue ram soon so I will pay about 15 dollars for one of them.
I've only paid up to £7 for fish (was £5 until a couple of months ago)- in US dollars that was about $14 a few months ago but the way the pound's sliding now that's more like $10-$11- 3 lemon barbs for £21, plus £7 store credit for a baby L191 Panaque (which was listed at £11.25).
Now I'm selling on bristlenose, Geophagus, Tilapia etc that have bred I've got loads of store credit so I can now buy more expensive fish if I want, but I prefer buying fish as juveniles and growing them on where possible anyway. Britain seems to be more expensive for most fish, but we've also got more choice as there are loads of aquarium shops within 20 miles or so of my house.
The most I paid for an individual fish was about $8 each for a pair of unidentified plecos, which turned out to be L387s. I think I got a pretty good deal since I can't find them sold anywhere since.
The most I paid for a group of fish was about $85, which was 20 Diamond Tetras. Fantastic deal! They are so cool.
I'm planning a community tank that will have some fish that will cost between $10 and $20, but I'll probably have to order them online, so you can tack on at least $20 for shipping.
I'd love to get some of the more exotic plecos, but I just don't have the room right now. I'm a fairly new fish convert (less than 2 years), so I'm getting there. I still don't think that I could see spending over $100 on one fish though. I'm cheap.