On the number of devices they sell, an extra $64 of cost per device (taking your higher figure and assuming an extra 8GB), across Mac, iPad and iPhone, they'd be looking at a cost of ~$12.8bn a year. If they just did it for Mac, it's still in the region of $2bn/year.
Sure they could pass that onto a mostly price-insensitive audience, but they like round numbers, and it's not the size of decision you take without making sure its necessary: that your customers are going to go elsewhere in either scenario of doing it or not doing it.
Sure they could pass that onto a mostly price-insensitive audience, but they like round numbers, and it's not the size of decision you take without making sure its necessary: that your customers are going to go elsewhere in either scenario of doing it or not doing it.