so how long is something out before it starts coming down in price?
about a month generally before prices have their "early adoptor" tax removed. But that depends on availablity then. If stock is hard to come by, prices could go up as well.
Expecting a 20% is proberly not going to happen, and even then it will be a shop based sale more than a across the range price change. For this country (Non-US), it will be after chrismas at a guess, and given availability of hardware then, proberly late janurary or early Feburary.
if you want a $100 price drop on boards, I think you are going to be dreaming for a long time.