I work retail and I can tell you that places like Best Buy, CCity, Compusa, etc. have very little markup on the computer itself. It goes anywhere from 5-10% depending on the brand and model. Retail stores make their profits selling you accessories, installations, and warranties, not on computer hardware. Places like ibuypower and Dell are different because they get to cut out a lot of middlemen (Distribution-Wholesaler-Retail-etc.) all of which add markup and they actually make decent money on the stuff they sell. But for a lot of computers I sell to people my employer makes about the same amount of profit on a $600 computer as a $30 usb cable.
So if I sold an HP computer for $800 it would have maybe 8% markup. So the prift there is about $65 or so. When you account for all of the steps that it took to get that pc from HP's factory to your home, however, that's a lot of lost money unless I can sell some extra stuff with it.