janharders - 8:37 pm on Jun 4, 2009 (gmt 0)

I'd say it depends on the niche and the quality of the sites.
If the niche is something where users will want to compare offers, the difference between the first and the following spots won't be that big.
If it's informational and the first site is bad, the next one won't be that far behind, because users will continue until they're satisfied.

I don't really have any hard data, because I don't know any real trusted figures for the monthly volume of the keywords the sites I'm working on are ranking.