According to fangraphs, he's already been worth about $300M in the previous 6 years. It's not far fetched to think he can replicate that for the next 6. Then you just have to decide what he'll be worth for the remaining 4-6 years. His glove will always play. He takes care of himself. Even if you factor that his production shrinks to a 1/3 over the final half of the contract (age 34 - 40), he'd be worth about $100M for 6 years. That's basically $400M for 12 years in fangraphs value.
You're earning a lot of value on the contract in the first half. You can't expect to hose the player for the last half. He's already been hosed enough. Through 2020, he'll only be paid $60M for his whole career. According to fangraphs, he should be worth almost $350M through 2020. Why does he have to eat that loss of approx $250M in excess value he's provided to the Sox? Why is it that only the owners can get ahead?
Mookie is the face of the franchise. He's a future HOF. You pay him and build around him.