Interesting point, because yes, generally if you buy something at a store and the next day it goes on-sale, they're usually good about making it up to you. Of course, as tinyE states, don't wait a month.
I've found that just asking sometimes works wonders. Not everything is set in stone, and stores are a lot more flexible than we give them credit for. It's just that the majority of people go in and get what they want at face value, so they (the stores) can afford to step out of the box now and then for those who request parley.
Of course, in the digital realm, with near-constant sales and, let's face it, pretty decent prices already, I can't see it happening much.