If we ignored the AMD launch altogether, I would reasonably expect nV's x60 parts to perform on par with the 580 for ~$200-$250, the x70 to be in the 50%-60% faster range and the x80 to be in the 75%-100% faster range. I had expected pricing to be in line, give or take a bit, with this generation.
That is simply based on the reality of Moore's law and it's actual premise being seen this generation(a roughly doubling of transistors in the same die space). Unlike CPUs, graphics scale almost perfectly with proportional increases in available resources(yes, edge cases like multi displays using high levels of AA will be bandwidth limited).
Since AMD has launched, I expect the pricing to be scaled up significantly and the relative performance dropped quite a bit(lower target clocks, higher yields). It's a fairly reasonable assumption that they are going to have a rather significant edge over what AMD has shown based simply on the realities of progress in this industry, but market realities and business sense will almost certainly having them lowering all of their targets unless they think they can 'take them out'(not likely at all).