I've seen this article pop up a few times now.

Assume conservatively that your True Fans will each spend one day's wages per year in support of what you do. That "one-day-wage" is an average, because of course your truest fans will spend a lot more than that. Let's peg that per diem each True Fan spends at $100 per year. If you have 1,000 fans that sums up to $100,000 per year, which minus some modest expenses, is a living for most folks.

Get to it, folks.