In theory, SEO is free. You can’t pay for a particular ranking. You can’t get a Google subscription and show up first in Google rankings. However, SEO is a mix of campaigns based on different strategies to take your business to that top position. If you do it yourself, it’s free to a certain point.
If you truly want results, an SEO consultant is your best bet because they have the experience and knowledge to handle such tasks. Even if you learn and do things yourself, there will be stages in SEO where you’ll have to invest some money anyway.