Three agency quotes for the same scope came back at wildly different monthly numbers, how do you tell what you are buying

Ask each of them one question: who does the work, and how many hours a month is that person on my account. A retainer is hours plus tools plus margin, nothing mystical. At a blended rate in the low hundreds, a 4k retainer is something like twenty five to thirty five hours a month, which is one good specialist for a day a week. If they will not give you an hours number or a named person, the answer is usually offshore contractors and a template.

Your problem is not really a content problem, it is a crawl and template problem, and those are unusually front loaded. I would push for a fixed price engagement for the technical work with a defined finish line, then a smaller monthly retainer for content afterwards. Agencies hate this because it makes the expensive part finite, which is exactly why it is worth asking for.

Cheapest quote is usually a content mill, most expensive is usually paying for account management you do not need. Get the deliverable list in writing with owners and dates. If they cannot describe what happens in month one at the ticket level, they have not thought about your site.

I paid a mid tier retainer for eighteen months and got four blog posts a month and a monthly deck. Nobody ever touched internal linking, nobody fixed the pagination that was eating our crawl budget, and the reporting was a screenshot of the same chart with a different date. What it cost me was not the fee, it was the eighteen months. Put a ninety day checkpoint in the contract with a specific outcome attached, or you will find out too late.

Politely disagreeing with the hire an agency premise. For 4k a month I would hire one experienced freelance consultant for six or eight hours a month to set direction, and spend the rest on two good writers who know your category. We did that for a year and the difference against our previous agency was that the writers actually understood the product, so we stopped publishing filler. The consultant told us what to publish and reviewed the internal linking, and that was enough.