It's a tough question. The Intel Atom has an edge on ARM, but it's not a big one, and while a high-performance ARM chip costs below $20, the Atom is significantly more. On the other hand, right now there are no ARM implementations that are really competitive on the PC front, and probably won't be until ARMv8 (64-bit) chips, or at least until Cortex-A15. A15 chips will probably come out in late 2012 and be a bit faster than the Atom, but a long way from Sandy Bridge and the other current Intel designs.