i wouldnt expect too much of a drop at all.. probably 10-20 dollars maximum.. they've both already had price cuts... nvidia's cuts being extremely massive... the gtx260 started at 450 dollars... and is now 300.... 150 drop in like 2 months.. i wouldnnt expect much more
cards are really overpriced on release, for them to make money off ppl that want the best of the best straight away. its always best to wait till at least the first drop in price.