The fairest way for booked performers would seem to be door money, which would reflect the ability of the performer to pull in extra people, with "a guarantee, to keep the organisers on their toes" as greg said. And that latter should perhaps be a flat, and openly stated rate, to avoid a downward cost-cutting spiral.

I think there's a distinction between an evening with a booked performer whose the main act who people have come to see, with a couple of floor singers or whatever to warm up, on the one hand, and, on the other, a regulars' session in which there might be a few extra spots for some guest who maybe had come some distance, and is there for free or for beer money and to build up experience.

There are two ways to look at this - one is the one in which the bottom line is making the club pay, with performers and punters seen as the way to achieve this, and the other is the one in which the bottom line is keeping the music alive, and that includes keeping the performers alive and active. Two sides to the same coin.