Jason Bay signs for the Mets; will it be worth it in 5 years?

So Jason Bay has finally ended the will he won't he by signing a 4 year $66 million dollar contract, with a $17 million dollar vesting option. Bay is a good offensive player, but is pretty awful at defense, though his numbers may be exaggerated a bit by Fenway. However, for him to worth, effectively the $83 million dollar contract, he'd have to worth about 2.5 WAR at least, going forward. And the problem with this is that Bay is not getting any younger; 31 this year, he'll be 36 at the end of the contract in 2014. And while I'm happy that the Mets got Bay, instead of going for the cheaper, worse option, I still think Omar somewhat overpaid him. Bay was not really wanted by any other teams, as the teams that were apparently interested in him, the Mariners and the Red Sox had both just made acquisitions (Bradley for the M's, Cameron for the Sox) that meant that they weren't interested in him anymore. Taking a 5-4-3-2-1 weighted average of his WAR since 2005, Bay averages 3.1. He was 3.5 WAR last year, and using CHONE offensive predictions and Beyond the Boxscore's defensive predictions, he is worth 3.4 WAR, with a .385 wOBA. He'll be paid $15 million in 2010, and a 3.4 WAR would be worth $15.3 million. Hopefully, he can keep this level of production up, but if he declines this deal can be very very nasty for the Mets, especially as Bay is owed $16 million up to his final year. However, if he continues to average 3.1 WAR 2011+, that would add up to a 15.8 WAR, which would be worth $71.8 Million dollars total, at $4.5 million a win. I'm fine with that, and if thats the case, it could be a good signing by Omar.
Now, go out and sign Matt Holliday to play right. That would be awesome.