A computer dealer can sell an OEM version much more cheaply than retail, but legally only if with a computer or parts. I picked up the full version of XP Home for 12,000 yen (about $120) when I bought my MacBook Pro at the same time, and this seems to be the average. They also offered it for 13,000 yen if you bought it along with an internal floppy disk drive, which was the cheapest PC part that could be bundled to legally sell the OEM version, at least in Japan. It might be different in the US legally, but definitely check with a PC parts store in addition to Best Buy.