The Fractal Design Focus G and Rosewill Stealth are probably two of the best cases currently available under fifty bucks. If you could go as high as seventy you would have a MUCH wider range of quality selections to choose from, but for fifty bucks or less those two cases offer pretty decent features including ones not really found on many budget cases like half decent cable management, fairly decent construction and an adequate number of case fan locations which is something most budget cases seriously skimp on.
The Rosewill Stealth is normally a 70 dollar case, but Newegg is running a sale on it right now if you use the promo code found on the Newegg product page. Don't know how long that will last though.