SEO is free from some points of view, but it comes with a premium if you need to do it by the book. It’s free, and you can learn to do it yourself. It’s also free as in you don’t need to pay search engines anything to show up in search results, even if you’re already in a top position.
But then, it’s not free when it comes to doing it right. Although there are quite a few tools out there that can help, it’s a consultant’s experience that you pay for. Mastering SEO strategies could take years otherwise.