That signs Upton through his age 32 season, so there is not likely a huge looming cliff at the end of the contract. Further, FanGraphs estimates Upton has been worth at least $15 million every year of his career except 2009. With inflation, should he continue the path he has already been on, this is likely a solid deal for the Rays from those estimates.
With the Texas Rangers' star centerfield possibly headed elsewhere, a solid deal given to a young CF may indicate some passers' remorse, but it probably shouldn't in this case.
For one, Bossman Junior is less helped by Rangers Ballpark in Arlington than Average Major League Baseball Player is. Righties get a boost by hitting in the Ballpark, but significantly less of one than southpaws. Particularly when it comes to home runs, where Upton gets a significant portion of his value from. It is also worth noting that RBiA exasperates the flaws of a high-strikeout hitter, though only slightly.
On top of that, the approximately $10-20 million a year Upton has been worth as a Ray is based on Upton's value above a replacement-level player. The Rangers' CF option is likely better than that. A few weeks ago, it was estimated a Craig Gentry/Leonys Martin platoon might well be worth better than three wins more than replacement-level. That's simply based on Gentry's career against lefties, Martin's projections prior to destroying AAA in 2012, and a conservative estimate of their defensive prowess. That may well be shorting their value.
In that case, while Upton represents a likely improvement, and probably a much higher ceiling, he does not likely represent enough of an improvement to be worth $75 million to the Rangers. Gentry and Martin are not going to be able to simply relocate to another position and still represent the same value.
This deal works in a vacuum, and it would not have necessarily ruined Texas. While it would be nice to see centerfield improved, at this cost, though, it is probably best to roll with what you've got and put the money elsewhere.