It's somewhat easy to measure various aspects of a PPC campaign and structure your fees around ROI; however, how many of you are confident enough in your organic optimization abilities to base your SEO fees around placement results? Would you base your whole fee on, let's say, top 5 results for competitive keywords in a client's industry?
If you raised your hand in response, then I want to talk to you ASAP. I have an immediate need for 2 top SEO folks with a company who DOES build in performance incentives to their SEO contracts. BTW, we'll pay you for your work- it won't be contingent on results. BUT, you've got to walk your talk.