What represents value and what doesn't is often complicated in the NFL. That doesn't stop people from trying to quantify it, though. That's exactly what Andrew Healy over at Football Outsiders did for the latest free agency period. Healy looked at contract vales and age-adjusted performance for every free agent signing, and tried to sort the good from the bad deals.
That method is necessarily vague. We have no perfect way to evaluate past value, let alone adjust it for the future. But it's as good as any method we have, and it's another data point in the larger process of figuring out which team did the smart thing, and which team did not.