I would wager to say that most people who are comfortable buying a new computer are equally comfortable building their own... so whatever you think it's worth, see if you can build a new computer for the same price and if the new computer is faster... well, you know you're overpricing your rig. I know 50% depreciation is a tough pill to swallow, but that's just the way technology works.
The month the Nvidia 7800 GTX came out it was $600 and I bought one at that price. Now? Ha... I wouldn't pay $20 for it.