Over at Baseball Prospectus they have a stat called Wins Above Replacement Player (WARP). It's a way to gauge how much a player has contributed beyond what the theoretical "replacement player," a guy plucked off the scrap heap or out of the minors, would do. The biggest flaw is that the defensive portion comes from using BPro's defensive stats which are pretty suspect. It will do for this though since I'm looking to see the general amount of value the players have each contributed, not get a definitive conclusion about their performance.
I'm actually using WARP2 for this since it adjusts for more than WARP1.
Interesting stuff for contemplation, if not the most accurate measure of their contributions to the team winning or losing.