Personally, I just built a new computer and I wish I had spent less on a case. I got an Antec Sonata for $100. Its tagline is "legendary silence." Well, maybe it's kinda quiet, but that's because they recommend only running 1 fan and only have room for 2. I ended up sticking a 2 slot fan next to my 4850.
The biggest problem with most "cheap" (<$50) cases is the junky power supply. On my old build, I got a $30-50 case (don't remember exactly) and the PS died in 2 years, I think. If you are going for a "cheap" case, try to get one w/o power and grab a decent PS. The Sonata PS better be going strong in 2+ years.