Whether SEO is free or not is a matter of perspective, and how you get your SEO services implemented.
If you do SEO yourself, it will cost you time and effort. There are many free SEO tools. But ultimately, it will be a strain on your time and patience.
Some say the organic search traffic is free. This is true if you are lucky enough to get 1st-page search results doing no work or putting no effort into SEO. This simply would be so rare, that you just as well play roulette.
Page 1 search results in the organic search are highly valuable because once you are there, typically the amount of revenue you can create is far higher than the costs of your SEO service. But it does cost to get there and stay there. From an agency perspective, no search traffic comes free.