Wes Welker is a good side by side comp. with Cobb. Same height/weight/speed. Used pretty much the same way. Was never the primary reciever. And Wes played the majority of his career with Brady and Manning. So the QB comp is there as well.
Wes ended his career with 50TDs and 9k yards. He earned a total of $41 million during his career. If you would have asked Tom Brady in 2012 if Wes was worth his production (he only hit double digit TD receptions once in his career, none with New England) Brady would have looked at you like you'd never watched an NFL game. Manning would have done the same.
Cobb has earned $24 million total at this point in his career. If he plays out the remainder of his current deal it becomes $42 million total. And he'll be in the same neighborhood as Welkers career stats. In four fewer years.
I know you never pay future money for past performance. But Green Bay has made a damned killing off the numbers Cobb has put up his first six years while paying him an average of $4 million per season. Including all the reasons mentioned by Boris.