Most expensive: My bristlenose plec, which was $25
Cheapest: A couple of my Cory's. They were on sale for $2 apiece.
Best deal i've got though was a fish that i'm not exactly sure what it's called. It's roughly about 1 inch long and probably 1/4 wide, with pink and black stripes running horizontally on it. I bought them the day after they came in. The manager told me it was a rare fish to find around these parts. Usually they like to hold onto fish they just order in for a week or so just to make sure they have nothing wrong with them, but i'd dealt with the manager many times before and he said he can make an exception and sell me one, although since they had never had them in before he wasn't quite sure what they were gonna charge for them. So we agreed on $12 and I took him home. Anyway that was about a month and a half ago, and I was in there today looking around and I happened upon a tank with a couple of them still left, and they're now selling them for $30.