Roughly however much you bought it for maybe minus $100 or so. BUt specifically, a few questions
1. Whats the rev of that CPU?
2. Specs on the memory?
3. Make and model of the optical drive?
4. Make and model of the PSU?
Well then, to be consistent with my earlier post i'd expect to sell it for the ballpark of no more than 2K (to be fair of course, anything else is customer's fault). Naturally I will note that unless there's some funky shipping costs or taxes, I think you paid too much for it