Re: your $20 Mayall story. That seems like a really cheap ticket? Some of the gigs I play in the USA with no name acts have $20 covers.
Mayall is more than likely to be paid a set fee by the promoter of the show/tour regardless of 100 or 1000 punters showing up.
The money definitely comes incrementally from a run of shows - you need to be doing 7 to 10 in quick succession for it to be sustainable IMO.
I heard a story that Nile Rodgers' guys are on $500 a show but that everything else is taken care of (food, travel, accommodation, instrument techs, dry cleaning etc etc). 20 shows a month and that's some serious cash going into your bank account back home!