I eventually learned the best model (for me) was to copy what Yoast did… charge a flat free for SEO Audits. People were always super happy with my audits. I still have my SEO audit templates (one for local SEO, national SEO, etc). Sometimes it would only take me 4 hours to write an audit and I would get $750, sometimes more if they wanted a more thorough audit. Maybe I undercharged?
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.