Ask five providers what SEO costs and you will get five pricing structures, not five numbers: a monthly retainer, an hourly rate, a per-article fee, a software subscription, and free-if-you-do-it-yourself. None of them is wrong. They buy different things, and comparing them starts with converting each into the same unit — the cost of one published, maintained page whose results you can tie to revenue.
The ranges below are commonly quoted ones, useful for orientation rather than quotation; scope moves prices more than any other variable, so treat any precise figure you are given as an opening position.
Two constants hold across every structure. The visible fee is rarely the whole cost — briefs, review cycles, uploading, and upkeep hide inside every option, including the free one. And the option that looks cheapest per month is frequently the most expensive per page once your own hours are priced in. This article walks each structure in turn, then closes with three questions that make any offer comparable to any other.