What do you recommend for a affordable guitar Mini Amp?

I have a Marshall MS-2 mini-amp. It fits your price range, and is comparable to the Fender mini-amps. Of the mini-amps I've tried in that price range (mainly Marshall and Fender), I couldn't tell much difference in sound or quality. I wouldn't expect a great tone out of one of these, though. They're more of a novelty than anything. Most run on one 9-volt battery, but all that I've seen have an optional AC power supply. It won't be a very loud amp (don't expect to be able to play over drums or horns), but it is really handy for practice. If you want a more serious amp which will deliver real tone in a compact form, try the Roland mini-cube. It will cost quite a bit more than the other mini-amps, and isn't quite as portable, but it sounds much better.