I would make sure you know what is being worked on each month to justify the cost. For example, I offer a flat fee that is the same for every client no matter what. That includes writing 3 blog posts, on page SEO changes, slack + skype access to me for quick questions, images to use on the site, forecasts for traffic gains, reporting, conversion reporting, and more.

If they offer vague things like “i’m building 5 links each month b/c links are good for SEO”, I would say this is not worth the money

This is just my opinion, but if you are paying someone to work on your SEO, you should clearly understand what you are paying for and what that will do for your business over time