But the flip side of that is that if you're a multi-millionaire, the sort of fees that would be reasonable are also kinda irrelevant.
Like, most of us would think £10,000 to come out and do two songs would be a hell of a payday. Springsteen wouldn't even notice it. He's pulling in over $1 million a night on tour. There's no fee McCartney could offer him, taken out of the already lower than average Glasto fee, that could make it worthwhile for him.
So at that point, even if he's being paid a few grand, he's essentially doing it for "free" and so often at that point they just don't bother with fees at all.
New Moor Mother is brilliant. Powerful, chilling stuff. Spoken word jazz music with themes of Europe's colonial & racist history. Real emphasis on British history too. Definitely worth a listen.