I assume we are talking factory overclocked Asus 570 Cu II not the reference model ? Here's a comparison w/ the reference model
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
$ 215.00 560 Ti - 900 Mhz (495/862) $ 0.43 - $ 0.50
$ 340.00 570 (524/873) $ 0.65 - $ 0.78
$ 500.00 580 (616/953) $ 0.81 - $ 1.05
On average, in the games listed, the 2nd GTX 570 gets ya 67% more fps for about 78 cents per frame. Guru3D didn't test the Asus Factory OC'd 570 model but looking at other sites, it averaged 13.0 % faster than the factory overclocked 560 Ti's in a single card configuration. However the 560's scales better in SLI (74% higher), so two factory overclocked Asus 570 CU II's are about 8.9% faster than two factory overclocked Asus 560 CU II TOP's. That 8.9% comes at a price increase of 48% tho.