It's an interesting question, and possibly one of those where the answer is what you are happy with.
In general, I would work on the principle that your level of pay is directl;y related to how seriously the payer takes you (taking into account market forces). Here in Pompey, the level of pay for pub gigs is pretty much what it was 40 years ago. There's something inherently wrong there.
In certain circumstances I am more than happy to pay for nothing, and do regularly. A commercial enterprise should not expect me to entertain their customers for nothing.