I've turned down a lot of gigs recently on the principal that the person booking the gig does not understand I would rather be paid than do it for 'exposure'.
The first line is always "How many tickets can you sell?" or "How many people can you bring?". I ask if there will be a set fee, which there never is, then am countered with "You get £1 back for every ticket you sell!" so they get to keep the remaining £4 and then in effect it has become pay to play...