I would guess 10-20% back, but that is just a guess based on the most that I would pay if I was crazy enough to offer them.
The colors would probably be more like less than 1/2 that of fancies.
From the information I have, they're asking about 20% less for HPHT whites, and 40% less for HPHT fancy colors.
I haven't read anything yet on how well received the HPHT stones are. I know that one retailer I spoke to that carries HPHT fancies says that they haven't been selling very well for him "yet".