When you said you need a "CPU" i got all excited to come in and talk about CPUs.
Then i come in here and find what you are looking for is a cheap tower. I wanted to talk about CPU's which will cost more than the entire thing you are looking for ! :P
Tower does not = CPU
Right now hardware as cheap as ****. Don't buy anything more than 2 gens back, as that shit sucks. You can get previous gen Pentium builds for a couple hundred bucks.
For prebuilt towers, try 3btech.com or Newegg