Gaming how much is my computer worth...?

[Splych]

alright, so my dad wants to buy my computer mainly because it's for his work and he needs a new one .
he said that since i spent my money on this computer , he is willing to pay me back for the same price i paid for .
but it is used and the price of parts went down . so since i have absolutely no sense of pricing things, i was wondering if you guys could help me price the computer:

CPU: AMD Phenom II X2 550 BE [clocked @ 3.1GHz] [http://www.amd.com/us/products/desktop/pro...comparison.aspx]
Mobo: Biostar TA790GX 128M [http://www.biostar.com.tw/app/en/t-series/...on.php?S_ID=377]
RAM: OCZ DDR2 PC2-6400 800MHz [http://www.ocztechnology.com/products/memo...gb_dual_channel]
PSU: OCZ GameXStream 850W [http://www.ocztechnology.com/products/powe...wer-supply.html]
Case: Cooler Master Elite 334 NV Edition [http://www.coolermaster-usa.com/product.php?product_id=2909]
GPU: Sapphire ATI Radeon HD 5770 1GB [http://www.sapphiretech.com/presentation/p...101&pid=341]

Harddrives, we are just switching them and Optical Drive, doesn't matter much. i think we are just gonna settle a small price like $10 to $30 depending on how the whole computer is priced at. so yeah, he was also thinking of buying a AMD Phenom II X4 CPU, but then i brought up the thoughts of unlocking the other two cores... i just haven't thought about doing it yet and also, i am not sure if i should risk losing CPU Temperature Monitor.

 

[Originality]

A rough guestimate, around $380-400 for the listed parts. As a rule of thumb, "used" parts tend to sell 10-40% lower than retail value. Naturally the most valuable part listed is the graphics card.