Assuming you are correct, it's a simple Supply and Demand issue likely.
When stock starts to dry up some people might start grabbing an HD7870 for Crossfire. My GTX680 from Asus was about $500 recently and the Asus GTX770 which is basically identical was $320.
Some people might just ask ridiculous prices knowing some uneducated fool will buy it once they are no longer in stores so they can't compare prices.
There is some price fluctuation due to a difference in QUALITY between brands though, and in some cases they may put more Video RAM on a card and jack up the price so people see a higher number (oh "4GB is better than 2GB right?").
Finally, some stores have completely different prices. I saw some GTX750Ti's for $150USD at...