I've also heard from some performer friends who do this sort of thing on a regular basis, and their working percentage is 50% (variable as inclination and circumstances dictate). There is a clear understanding in writing as to who is responsible for which aspects of the production; i.e. promotion, sound, venue, etc. CD sales are also covered with a portion being donated back to the fundraiser.
Biggest problem now is my own personal inertia! I'll post further should I manage to get this off the ground.