The newer gen brings less power consumption, less heat, and of course a higher price
the 7850 only outperforms the 6850 by an average of 10%, not worth double the price of a 6850
I am not sure what your smoking, but the 7850 is just a little below a 570, right behind it in skyrim and battlefield 3, and its the replacement for the 6950, way more that 10% over a 6850. And they are CHEAPER due to the die shrink. That's how die shrinks work.
I would not get a 570 at this point. You have to wait for ivy bridge, wait for Kepler too. THAT will bring AMDs prices down. There's no guarantee 570 prices will go down when Kepler comes out or that you'll be able to get a second for SLI later. 6950s prices rose and sold out quick when the 7000s came out and manufacturers switched over. All the people wanting to crossfire before they were gone bought them up. And their replacement isn't even out yet.
The 570 is end of life at this point. But if you never want SLI and your happy with the price its up to you