In response to the original question... yes, there are compaines out there. However, I don't think there are many. I've actually seen this model more with Online Video Marketing than
SEO, and here's why:
The problem is that
SEO takes time - results aren't instant. Thus, it's hard to maintain that client-consultant relationship over months and years if the payoff (for both parties) is over a long period of time. This would probably only work if you get
SEO from your web hosting company (assuming they offer it).
Basically, you'd be better off using PPC if you don't want to pay the money up front (plus you can stop whenever you want, and decide your own budget).
A good
SEO company might charge thousands of dollars for a year's service. But over the next 5 years, your site might get more than enough income to warrant the initial expense.
That's why most
SEO companies won't offer that pricing structure.