The PPC experts at Did-It.com have created a Conversion Index that “measures the rate at which consumers convert into customers from pay-per-click listings across major search engines, their syndication partners, second-tier search engines, directories, and portals.”
What that means is it’s easier for PPC advertisers to take an aggregate look at what PPC solutions are driving the most total conversions, which have the highest conversion rates, and which have the highest click volume. It also looks like they’ll be trending the data over time which is interesting to see how they perform over time.
Obviously, since these are aggregate numbers you still need to test for your ads and your site. It’s quite possible that for your industry or for your site you get different results than this overall index.