Our (Friends of Acoustic Music society)open mike charges $1 a head, which covers room rental, with no charge to anyone who gets up to share -- er, perform. Naturally, everyone, mostly, comes up with a little something to do.

The local duo of Wink Wink started an open mike at a (very smoky) tavern which is very very well attended. no cover. People are glad for the experience, you do have to sign up first thing (which means I never perform there because I work until 8:00 that night), and a couple of people have ended up cutting very decent CDs with the local studio (Satellite City). Every community is different.