Thank-you both. So, Wikimedia and a fistful of Linux-related projects all manage with charity, basically. There will be a cost-level where donated labour and mere charity are sufficient funding.
Much of the Fediverse may need only that - a little guilt-tripping with posts from your server admins that resemble Wale's pleas on the wikipedia main page.
Then there's the PBS model - corporate sponsorship for a modest acknowledgement, i.e. minimally-annoying ad.