This. Also, there is a possibility Apple is willing to forego some profit (make only 70% rather than 100% profit) as a strategy to seed the market in order to give developers an incentive to adopt ARM faster.
They already have distributed cheap developer machines.
And their profit on Mac hardware is only 15-20%. You may price at double build costs, but there is still design, R&D, testing, sales, marketing, distribution, support, warranty costs and admin overhead to be accounted for.