What is a good selling price for my used computer? It was Custom Built in 2007.?

I would say a great deal for an 3 year old computer but value of stuff goes down and maybe if updated OS you might get half what you paid into but with what you got be likely to get $550 as is w/XP with Win 7 might get $750, of course WIN 7 Pro kinda of costly but it would make improvement.