either that or the sellers are smart and play the market... they sit and drop prices very minimally wile people wait for a large price drop... when it doesn't come people start to accept that fact and simply buy at the inflated prices... once this starts to happen, they drastically drop prices and decrease stock for a few months and then let them naturally skyrocket... since bit mining is based off of pure cores and not how effective they are (as we all know), the 6xxx cards are going to be more effective than the 7xxx cards because new generation go for better efficiency with slightly more raw power on the high end, not brute force all the way through... the 6xxx card will jump to their premium and the 5xxx cards (because of their brute force style) will sell for even more... once this happens, sellers start to release their stock slowly (just like is happening with the 58xx cards right now)... naturally, the sellers make bank wile the buyers suffer, but that shouldn't stop miners from buying the cards (especially as BTC are more readily accepted)... if i had to guess, the price of a 5870 will jump to $300+ if they play the market well enough
this is just my take on the whole thing... any opinions on how this might work out? if you see any flaws in my analysis, please point them out as i like to know when i say something false so i can correct it the next time
umm this is all speculation right now, and you're most likely going to be wrong on the part that you said 7xxx cards will be more efficient, but less brute power than the 6xxx. the reason you see the 6xxx run faster than 5xxx cards per stream processor in games is because this generation jump does not have an architecture die shrink. both of these generations still run on 40nm die, so in order for a 6xxx card to be "better" than the 5xxx is to have more efficient stream processors, so that's why they can afford to have less of them. of course, this will hurt when they're used only for mining.
however, with the 7xxx cards, it's a complete architecture change, the die will be shrunk based on 28nm tech. with a die shrink, they will be able to fit more stream processors in the same amount of area as the current generations cards, and it will most likely also have some computing and power consumption efficiency improvements. the key here is a die shrink, so it will most likely have more
stream processors than 6xxx, not less.