> I would be shocked if an instance cost more than $40/month to host, and I'd reckon the typical instance is less than $20/month.

That information is public for at least one moderately popular instance, fosstodon.org. According to https://hub.fosstodon.org/about/#current-funding the current cost is $1895.50 per month, plus $200.00 for the CDN. (And yes, last time I looked, their Patreon is going to be more than enough to cover that.)