Pay-as-you-go SEO service?

I'm looking to outsource a bit of SEO, but I'd like to switch over to using one centralized service rather than multiple freelancers. I remember reading about a pay-as-you-go ranking service a while back. Their website had a keyword analyzer tool which would tell you how likely it was that they'd be able to rank your site for that keyword. Anyone know what site that was?

Thanks!

Edit: Found them: http://www.rankpay.com/

Has anyone tried them before?

interesting concept to say the least. Also looks like the prices are rather competitive too.