It's already a FAQ from (wannabe) mirror hosters, so we should document how much it costs to host a mirror.

That depends on a lot of factors and we do not want to endorse a single specific way of hosting a mirror, nor advertise a single provider of hosting solutions. So one way to go about it would be to describe a handful of use cases and, where external service provides (e.g., public "cloud" vendors) are involved, include pricing from several vendors. I can imagine at least two macro use cases—self-hosting v. "cloud" hosting—with a few sub-use cases, e.g., scale-up/scale-out for self-hosting and at least the 3 major hosters for the other macro use case.