As to compensation, a sculptor sometimes has the benefit of being constantly compensated for the sales success of their reproductions; I cannot ever mold and mass-produce my paint jobs, each one takes the time it takes, and is potentially unique even in the case of when I paint two identical models (the same sculpt reproduced). It's quite often an apples and oranges result. If a sculptor doesn't get what they feel the work was worth, it's not on me to feel guilty about what I get paid to paint it, though.
As far as GW goes I believe the sculptors are paid a flat salary and do not get a commission of sales, but I could be wrong.