Conversion rate depends on so many factors including price of site, that it is just about meaningless imho.
Of course more people will sign up to $25 site than a $39.99 site, so conversion rate will be higher, but either may be more profitable to promote.
The only statistic that I focus on is $ earned per banner click which is calculated and ranked automatically by
Stats Remote (if you wanna see my stats then check sig).
Having said that, my average conversion ratio on uniques is 1:1381, and my best converting normal paysite is currently
Scores Cash at 1:108.