Why would it be too expensive? How many fab's are there (so, which % of worldwide output does each do), and how much % of all intel chips is bought by Apple? If the numbers are somewhat equal, they buy the capacity of one fab anyway. Then they can just as well buy the fab itself.
I guess Apple buys less than 10% of Intel's x86/x64 chips. http://en.wikipedia.org/wiki/List_of_Intel_manufacturing_sit... shows Intel runs at most 6 or 7 plants that could do sub-40 nm chips. So, that would make Apple's part at most half a plant or so.
Finally, I guess that buying a plant is not sufficient. They also would have to license tons of patents. Without relevant patents to exchange, that would IMO be prohibitively expensive.
