There's no such thing as a standard pricing model with search engine optimization companies. This makes it difficult for site owners to compare. I often see requests for people wanting a Pay for Performance model, but I rarely see a company that actually offers this. There is a search engine optimization company that I am familiar with that offers pay for perfomance optimization. That company is eTrafficJams.
I've never used eTrafficJams before, but then again, I don't generally use another seo company's services because I don't need to. But most often, you can tell a lot by the information on a company's site. I've scoured eTrafficJam's site to find anything that would strike me as being wrong, but with the exception of "search engine submission services", it looks good. I always wonder why any SEO company would offer search engine submission services, but I have to assume that it's always thrown in there because the public expects to see it. And since it's almost always thrown in, I generally ignore that when I see it.
Their pay for performance model is based upon the client only paying for results, where they "only get paid for the Top 10 search engine rankings we achieve for you in the Three (3) search engines that matter most: Google, Yahoo! and MSN." Of course, my first thought was WHAT keywords? After all, anyone can rank your site for "super duper widgety thingamabob that spins and flies". So, I dug deeper and they say that the keywords are ones that are mutually agreed upon. Ok, I can't find fault with that.
Next, I read everything I could find on the site, including their services, testimonials, blog, articles and associations. Again, I couldn't find fault with anything. Finally, I ran some searches to see if I could find any negative reviews and didn't stumble across any.
So, if you are one of those people looking for a pay for performance model, you might want to check out eTrafficJams. As with any SEO firm you are considering, you should do your due diligence, and ask questions. Make sure you agree upon the keywords that they plan to optimize for. Make sure both you and the company are clear on the details, and then give them the leeway and time needed to produce results.
The above is a paid review of a website. I do not have any actual experience with them, nor have I ever used their services. I simply reviewed the site itself and based my opinions on that.