Fielder will reportedly play first base, while Miguel Cabrera will move (to DH or Left Field, presumably.)
The total dollar amount shouldn't be too surprising, although the years come as a shock. It seems like the only other serious contenders for Fielder were the Rangers and Nationals, and there were issues with both of them, so for Fielder to get so many years guaranteed is just another testament to Scott Boras' skills. Long term deals like this seem to work out rarely, although they aren't quite as bad for hitters as for pitchers. Fielder is young, at 27, which is a plus, but his large size also creates concern over how he will hold up.
So while this deal would look better if it were two years shorter, I'm gonna go out and say that in terms of dollar amount per year it's probably reasonable given the market for other first baseman. What's surprising is that the Tigers already have the best first baseman in baseball, Miguel Cabrera, as well as Victor Martinez. Martinez is out for the year, but has three years left on his 4 year, $50 million dollar deal. So the Tigers essentially now have three DH's under contract for at least the next three years, making big money (Cabrera's deal ends in four years, in 2015.)
Cabrera hasn't played the outfield since 2005 and hasn't played third base since 2008, and he was brutal at both (he's simply bad at first.) Meanwhile UZR rates Prince as a poor first baseman (comparable to Cabrera,) and obviously his body size suggests that he will continue to diminish. Martinez essentially DH'd all last year, and when he comes back he will be 35. So essentially the Tigers are going to have to take a bath defensively somewhere; Fielder is probably less valuable to them than he would have been to several other teams as a result.
While the dollars per year may be fair, the nine years and the current makeup of the Tigers roster seem to make this a problematic signing for the Tigers. At the very least though the Tigers should walk away with the AL Central, and if they can win a World Series everything else probably won't matter as much.
Shameless Plug Alert: Listen to Dave, Thomas and I be hopelessly wrong in predicting what would happen with Fielder.