Interesting concept, similar to VBD as Indibuck said.
I'm using a base VBD and giving each position a "relative value" - RB and WR are 1, QB 0.75, TE 0.7, Defense 0.6, and K 0.5. Not sure i'm sold on that system, but it does rank the players about where I'd guess they should be.
I might steal the first half of your equation, though, and see how that shakes things up.
"We couldn’t do diddley poo offensively, we couldn’t make a first down, we couldn’t run the ball, we didn’t try to run the ball, we couldn’t complete a pass - we sucked." ~ James Ernest Mora