ASICs are expensive for small quantities, cheap for large. But how many SHA256 ASICs would one expect to be able to sell? And how much performance per Watt can actually be achieved?
The power consumed by a 2GHash/sec ASIC would not exceed 5W; this is based on inquiries with friends who make ASICs in their day jobs, and assuming a relatively straightforward, unoptimized design. So in other words, power requirements would be 50 - 100x lower than with GPUs.
5 times lower than GPUs, actually.
200 MH/s per single chip in plastic package.