Last year I attempted to calculate the Packers' surplus value using my own valuations of each player's worth and their corresponding cap number (as opposed to cash or AAV). Readers did not nitpick my own estimations too much, and while I tried very hard to look for comparable players and otherwise be objective, still the valuations were really just my own opinions. So I was happy to see an alternative.
This year Overthecap is offering its valuation of the worth of almost every player in the NFL. OTC does not explain its exact methodology, noting that it is proprietary, but does indicate that it is based on playing time and PFF grades, among other tools.