SEO is not a shelf product with a pre-determined price. Each site, in each industry, with the competitors and their SEO, will carry a different prices. More than that, each digital marketing agency will vary in price.
We suggest you not to be tempted by cheap offers, as these are probably amateurs who may do more harm than good (unfortunately search engine like Google can punish websites and disappear them from the search results).
Just to estimate, prices for SEO will range between $400 – $1,000 a month, certain areas that are even more.