I run a comic book related forum with thousands of registered members, but of which only a hundred or so are active at any particular time.
In the order of donations, I run a yearly fund-raising effort (ala Wikipedia) in which I specify the target as the cost of hosting (~$800 or so, give or take), and I've yet to go a year where I didn't make that goal (though I haven't attempted to fund raise every year).
Communities are of course different from the average web app, utility or program, as communities tend to take care of themselves.