I'm pretty sure a computer with the exact same components as an X360 would cost quite a bit more than the X360. However it is not really possible to get the components since they are pretty much tweaked and customized to fit perfectly together and run as M$ wants them too. Then again, as Curly_G sais, M$ is probably not earning much from the actual console-sale, their main income will probably be from royalties and stuff like that from the game manufacturers.
So if you're really willing to sell your soul to Sata.. erm.. Microsoft.. then go ahead and buy the comfy "ready-to-play-kick-boat-games" console..
