I asked Lee Sklar whether he charged multiples of scale like, say, Will Lee. He said he doesn't.
Nile Rodgers splits every collaboration 50/50 regardless of the contribution.
If you look at what Nathan East has managed to work out...a million dollar home plus other properties...persumably from the success of Foreplay's touring and album sales as well as his sessions and wife's occupation as a paeditrician...he's done extremely well for himself.
We used to have Kylie's keyboardist from her Fever tours dep with us and he said his pay was pretty much the same as if he was in a function band. Just that he got paid for gigging every night rather than once a week.
I think its hard to tie down earnings, there are so many different potential sources from merchandising through to spin off work and marrying into the band.