4yrs $50 mil, equals $12.5 mil per season. The sox paid him $12 million in 2011 alone. He's getting roughly the same rate as the sox gave him. And he has performed at an elite level for his entire stay in Boston. The gold standard is $15 mil a season, which Mariano is earning. Papelbon isnt Mariano, but he's probably right behind him in performance over the past 6 seasons. He's also 30 yrs old. A 4 yr contract takes him through his 34 yr old season. He shouldnt see a non-injury decline in that span. If the contract took him past 35, then I'd agree. The contingency with closers is injury. Mo is lucky he hasnt had a major one in his time in NY. Papelbon had his shoulder scare, but he hasnt either. If he stays healthy, he'll be worth the contract. If he doesnt, then he won't be.
I've said it 1000X on here. Papelbon isnt irreplaceable. He's just very difficult to replace. He's given you elite production (sans 1 season) out of a very important position since he took the role 6 yrs ago. The combination of production, durability, and ability to handle the pressure made him worth what he is earning. If he continues to show he can perform at that level, then he's worth it.