I would think that any folk club would have incidental expenses even if they didn't book guest artists (and most do, I think, even if it's just occasionally). There is advertising, the cost of maintaining a website, cost of a room, equipment, phone calls, etc. I find the idea of anybody making a profit off of a folk club hard to fathom, and even if the organizers did make anything it would hardly compensate the time most of them put into it.