Affiliates are always looking for indicators on whether the affiliate programs they are promoting are performing well. One measure of affiliate program performance is the conversion rate.
I collected data on conversion rates for both per lead and per sale affiliate programs for the latest AffStat report on affiliate benchmarks.
Below are the conversion rates for pay per sale affiliate programs, as reported by affiliate managers.

These numbers represent a range of affiliate programs from small, niche companies to the biggest retailers.
More affiliate stats at http://www.affstat.com.