It doesn't matter how many millions they order, someone's keeping a manufacturing line churning out these anachronistic drives just for them, and it likely comes at a price. Even 40GB has had its time come and go. These drives (20GB, 40GB, 60GB, 80GB) are *all* single platter drives - it's simply the platters themselves that differ.
Just because the storage capacity is anachronistic doesn't mean the drive has to be. The HDs used in the X360 are made to order by Seagate. They could conceivable use the same parts as higher capacity ones. I remember reading an interview of the Seagate CEO in BusinessWeek, where he said that their cheapest platform is a 80GB single platter drive. By now it might be 120GB.
According to Takahashi, the quote Microsoft got from Seagate was $30 to$40--around the same as the cost of the drive in the original Xbox towards the end of the cycle.