Also: Not to be the guy asking the stupid questions here, but what is stopping bulk purchases of GPU chips (specific clocking/memory designed for mining) in bulk from AMD? With say 25 undervolted and finetuned 7850 chips on a fairly simple board that would plug via USB and be recognized as a multi-crossfire system I can see that being a $$ while solution. Or maybe I am just dreaming..
You need to have product volume in order to get the chip vendors attention.
FPGA you can go to any number of distributors and buy them one at a time if you like (with discounts at various volumes).