Won't comment on selling price. Will advise you to remove the harddrive and warehouse it.
Reformatting a drive doesn't do a thing to deter the curious and half-determined from lifting data off a "clean" drive. Unless you are sure there was never anything on that drive you care about someone getting their hands on, don't sell the drive with the machine.
If you're really concerned about people being able to lift data from a reformatted drive (and yes, it can be done), use a program to zero-fill your drive before selling your system; which is probably worth about $200-$250.
If you grab the ultimate boot cd, which you can get the iso from the web for free. It's legal as far as I'm aware, it has Darik's Boot and Nuke(DBAN) as someone suggested, it even has a Dept. of Defense strength data wipe on it, which should make it difficult for people to recover your info.