Like allthumbs said, about the best you're going to get for around 50 bucks is a micro-amp, unless you find a deal on a used one on e-Bay or in a pawn shop or something.
Both Fender and Marshall make micro-amps that look like their larger amps - "mini-stacks", etc. I've never heard one so I don't know how they sound. A lot of them run on 9V batteries.
Other than those, there are a handful of headphone amps in that price range. I don't know how any of them sound either, but with those you'd be completely "stealth" because the sound only goes into your headphones.
if you want to get an idea of what you're looking at in that price range, go to
http://www.musiciansfriend.com and click on "Amps" along the left side of the screen, then the "Mini-Headphone" sub-category. They show a pretty large selection there.