I'm guessing $500-600. You can buy a new, complete Sam440ep-flex system for ~$800, and the Sam460ep should be available soon. Folks will be looking for a bargain--something less expensive than a newer, better product, anyway--and they won't place much value on things they likely have sitting around their workspace (HDD, DVD+/-RW, chasis, etc.). With $500, you can build a nice i7 system. I think I spent ~$600 for my base kit (Core i7 920, ASUS P6T Deluxe, 6 GB DDR3) in 2009, and prices have come down considerably. (A good video card will be the system's most expensive component. I limit set my budget at $250, so I just bought an overclocked GeForce GTX 460.)
As with OEM copies of Windows on a PC, you're probably required to sell OS4 with the Sam440ep, regardless of your intent. Be sure to use Hyperion's transfer of ownership web app thingy after you sell it.