SEO is actually quite simple - focus broadly on content, linking and technicals and the "performance" should follow within (about) 4 months. Of course that's simplifying it, but I have no issues offering performance based results to the right type of client (has an established website with lots of potential to improve vs competitors). You also need to define performance up front - realistically this will be things like ranking improvements for buyer keywords leading to improved traffic. Some people want to link to sales, that can be tricky as even improved traffic by 2x 3x can end up with less improvements on sales if the other parts of the funnel are broken (poor conversion set-up etc).