I'd have to guess $700, simply because with good overclocking the performance should be comparable to two 6970s, which are still hovering around $350. I could see them moving to $600, especially if Nvidia is aggressive with pricing. Either way, the 6990 should force the price of the 580 downward.
Looking at the Anandtech bench comparisons between the 6970 in Crossfire and the 580 in SLI, the performance advantage of the 580 isn't as much as I expected, especially for the price difference. Perhaps its due to better scaling for AMD (or just the listed benchmarks being more favorable to AMD), but even if Nvidia releases the 590 based on GF110, I don't think they'll be able to command a significant price difference.