RAM: Depends on speed if it's PC3200 about $75
HDD: around $50 a piece depending on RPM
GFX Card: $30
Mobo: no idea...
Sound Card: under $25
This is assuming you've used it for about a year or so.
About $400 total - It's probably not worth selling, and the price is decreasing every day. Just keep it.
C2D E6600 | 4GB DDR2-800 | 9800GTX+ | Asus P5B-E | 150GB Raptor | 320GB 7200.10 | 750W Xigmatek PSU