$0. It was a gift. That is, of course, barring upgrades to the harddrive, RAM, upgrading to a DVD burner and removing the Zip drive and modding in a floppy drive. With that all, it would come to about $225.
Before that, $100.
(Yeah, it was an Athlon T-Bird 900mhz with 256MB of RAM in a $10 case, so what?)