Have you overclocked?
What is the age of the PSU and HDD?
What is the condition of the case?
But at a guess, $550 to $600. Though if the case is a bit battered you can knock $20 off, $30 if the PSU is older than say 2yrs.
Computers aren't like fine wine, they dont value well with age.
EDIT: If you have overclocked, especially if you'v increased the voltage you can expect another price drop.