The licensing cost for a handful of Nvidia-specific GPU extensions in no way explains the extreme cost difference between a standard 2.5" HDD and the Xbox 360's HDD.
Microsoft (as well as Sony and Nintendo) sells their consoles at a loss. By marking up the cost of proprietary accessories, part of the loss can be recouperated. People will buy it at that price, so why lower it when there is no real competition? --- Perfection. Where everyone fails. If you expect nothing you can be happy for everything.