There are a lot of factors that can affect the cost of optimizing your Shopify site for search engines. The size of your site, the competitiveness of your industry, and the current state of your SEO are just a few of the things that can impact price.
That said, there are a few general price ranges you can expect to see when shopping around for Shopify SEO services.
For smaller sites, or sites that are just starting out with SEO, you can expect to pay somewhere in the $500-$2000/month range. This will get you basic optimization and some keyword research.
If you have a larger site, or you're in a highly competitive industry, you can expect to pay closer to the $5000-$10000/month range. This will get you more comprehensive optimization and a more robust keyword research strategy.
Of course, these are just general ranges. The actual cost of Shopify SEO services will vary depending on the specific needs of your site.