This topic is trying to cover far too much ground. As I understand it, the OP's question referred to serious pro musicians who can charge a serious rate for depping because they're good enough, flexible enough, and (presumably) have some sort of reputation.
Well I've depped too. Twice. I'm not good enough, not flexible enough, and I have no reputation, so I got the same money that I'd have got for playing in Dad-rock pub band. Seeing as that's pretty much who I am, I didn't have a problem with that.
Bassfunk talks about charging up to £350 for a dep gig. Very few of the bands I've played in were capable of earning that for a pub gig, and that's for the whole band.
As to 'we should all sing from the same hymn sheet and ramp up the prices', sorry Si but that's simply nonsense in 2018. Musicians could get away with operating an unofficial cartel in the 60s and 70s, pre-Internet and pre-thousands of newly qualified music graduates appearing every year.
This is a capitalist country, and everything is driven by supply and demand. Supply has been soaring for the last ten years, at exactly the same time as demand has been falling. Do the math.