9 Years, $214M

Also known as "Mike Ilitch pulls a Ewing Kaufman" In nine years, the Prince Fielder contract will probably be a disaster. In five years, it may be a disaster. But Ilitch is 82 and he ain't taking Comerica Park with him when he goes. That $214M might as be a one year deal, as what Ilitch is buying is another shot at a World Series ring this year, right now, while he's still around to enjoy it.
In terms of a rational look at the numbers, no, it doesn't make sense. It's a loooong deal, Fielder's body type generally doesn't age well, it's a ton of money, and the Tigers already have a couple of slugging fire hydrants in Victor Martinez and Miggy Cabrera. Sure, Cabrera can stand at third with a glove on, but the man clearly knows the location of the Magical Fountain of Pie, and he's not giving it up for love or money. All three of those guys are owed a lot of money, and in the long term, if you're looking at the extended well-being of the Detroit Tigers as a franchise, it doesn't make a lot of sense.
But the thing is, we're not, or at least Ilitch isn't. His calculus is "I want to win now, and I will spend what it takes to do that". So he spent, and within that frame of reference it makes perfect sense. If the Tigers win the Series next year, it will have been worth every penny of that $214M to Ilitch and then some. And that's just fine.