This is the infamous Western Digital Scorpio Blue hard drive, that will fit into both the PS3, 360 and 360 slim (and of course PC), for just £26.58.
To get this working with the Xbox does take some computer trickery, but you really only have to just follow a tutorial.
Much cheaper then shelling out £75~ for an official one!
Delivery is free if you are an AVForum member, http://www.avforums.com/forums/scan_offer.php
Read thread for more details.