On the small business end, I've run a few interactive sites (but like 95% read, 5% write) with a million or so visitors a month and they typically cost a few hundred dollars (like $300-$500/mo). Most of that was for business-level (but not necessarily enterprise) support. The actual infra was much cheaper.
It heavily depends on how much you can cache and CDN, and how many writes to the DB there are and how real-time it has to be. It's dirt-cheap for something like a blog or ecommerce site (which can be like 95% static content), but way harder for a real-time discussion board, reddit/HN clone, etc. (I think there have been a few posts here describing HN's own infra and costs running on some homebrewed code basically on a bare-metal server in some office? Can't remember exactly).
One of my previous employers was collecting data from a few thousand IoT devices every few minutes. Our pipeline wasn't very optimized and it was costing us maybe $30-$40k/mo in AWS spend, if I remember correctly. I didn't work on that part of it though so I don't know all the details.
One of my other jobs was for a museum; we had a simple informative site that they were getting ripped off for (enterprise Drupal hosting, yuck). I rewrote that in a more modern stack and hosted it on Vercel for about 30% of its previous cost. Think maybe it was $500ish/mo for a nonprofit enterprise plan? It could've been even cheaper than that but they wanted the business support. Honestly it probably would've been fine on the $20/mo plan, maybe with a bit of bandwidth overages.