If you don't mind using the case over, and providing it's not proprietary like HP/Compaq stuff is/was, you should be able to rebuild with new stuff in the case.
However, at the cost of cases, if you rebuild, I'd just use a new case. New Egg has plenty with free shipping. Brian's ran for 39.99 at the time of purchase and had the free shipping.
If you need a cheap beater, I always tip my hat to Dell Outlet refurbs for a factory built. You don't have the overhead of a new system, and still get a warranty. Some will come without an Operating system, so it will have to be installed.