Over the Cap is a website devoted to salary cap related issues. One of the things they do regularly is to calculate a value based on production for each player and roster, then compare it to the player’s actual contract to analyze whether it is a positive or negative contract situation.
Here’s how they describe it in brief:
OTC’s player valuations are calculated using proprietary formulas to more accurately depict the value being provided by a player based on his on field performance relative to the current market for his position. The calculations utilize a number of statistic and performance evaluations that are provided by Pro Football Focus.