Two clubs I go to have session nights most weeks and guest nights less frequently, and yes, there is a bit of a problem getting some of the regulars at sessions to support the guest nights. We do have a small fee for session nights, which helps to subsidise the guest nights. At the one club we always pay an agreed fee, regardless of size of audience: when it has been very small, a couple of the artists have suggested that they would take a lower fee, but we have always refused this noble offer and paid what was agreed. There are a few floor spots at both of these clubs on guest nights so for those who "just want to sing" there is still a chance of one song if they get there early enough! On a busy session night you may only get one song anyway at one of these clubs.