Al wrote:-
"many major artists ask support acts to pay for the privilege of borrowing their audience"

This comment takes me back to the very first time that I heard of such practices - decades ago - and concerns a huge long 50 date tour that Billy Connolly was doing when he first hit the big time. Billy told the tour organiser that he would like a support act and that, with his background, he would like it to be a folk band. It was put to Billy that the support should be the Albion Band, who were just starting and would probably like the exposure. "Good idea" says Billy so the tour organisers get on to the Albion's agent who asks how much and is told £x (I really can't remember the amount) per gig. The agent puts the deal to Ashley Hutchings.
"£x?" says Ashley, "It's not a lot to split between the band.... but we would be working every night to big audiences.... and we have a new album out which we can sell at the gigs.... I wonder if I can the record label to pay the hotel bills and then it might just be worth it."
So the Albion's agent goes back to the tour organiser saying they might just go for it and gets the answer, "Right, £x times 50 gigs - that's £y. Put your cheque in the post and we'll get their name on the posters."

I heard this story from more than one source but have no idea of the truth of it. I remember thinking that it was an urban myth when I first heard it, but as time has gone by, I am not so sure.