To accurately calculate depreciation take the amount you paid for it, subtract the salvage value (if there is any) and divide that by the life of the computer. Say you originally paid $1K for the PC in September and you've owned it four years and there's a salvage value of $200.
($1000 - $200 / 4) * (4/12) = $266
Your system is worth $266. Now depending on the condition it's in add or subtract another $50 to get the selling price.
to anyone that knows anything about computers worth about 100 to an average joe on ebay 200 2n hand computers with better specs are going for the 250 price. Sorry man I know it feels like it's worth more.