Hey, I build custom computers as a little side business/hobby.  Basically, I charge between 50 and 100 bucks depending on the labor involved, plus price of parts and shipping.  If you're just looking for a tower, i can get you something in the 800 dollarish range that will perform well, although I'd recommend spending more like 1200 (I'm not a fan of cutting corners, although I realize in some cases it's necessary).  You can email me at 
garasaki@aol.com if you're interested.  I can quote you a price...