Who knows, but the value depends totally on the demand for them or projected income for various projects.
As for pay, yeah it's private, but who cares? I'd have no idea what they make or how they're renumerated. I ber DG gets the most, seeming as he's the most senior, then SW, blah, blah blah. Then again, since it'd pretty much all they do, they may have to just take what they can get.
I dunno. Interesting thought, though. Filmwork would obvioulsy be the most lucrative, and sonce they haven't had a theatrical release in over 7 years...