In short, we can reasonably guess that an average SEO expert charges between $100 and $175 per hour. Further, the average monthly retainer lies somewhere between $950 and $2500 per month.
Corporate SEO and larger eCommerce SEO can run in upwards of $25,000 per month.
Performance-based SEO is free to start, and has dynamic pricing but a costly alternative to a revenue share plus attorney costs. There is always the start up fee associated with Performance-based SEO as well.