It has to be a combination of MULTIPLE metrics that are combined and weighted against the league average.
Again, just like in Baseball. It's obviously different, but it's the only sport where there is a respectable (and respected) Player Ranking System.
However, they DO NOT use as a way to judge how much a player should be paid.
Check out my link above, if you haven't already, where they explain how in depth and convoluted the Baseball Metrics are. It's pretty ridiculous.