Where it stands
Here are the ten (actually eleven, due to a tie for 10th) largest contracts in team history, together with the years covered and the WAR value of the player over those seasons, regardless of whether or not they were still a Diamondback at the time. Those where the WAR is marked with an asterisk, are still in progress at this point.
With a couple of exceptions on either end, most of these ended up as being okay, rather than brilliant or disastrous. The most notable of the former is probably the Big Unit's pair of deals in Arizona. which cost less than $2 million per win between them; at the other end, you find Russ Ortiz, though Montero's contract is potentially beginning to resemble a large-wingspanned seabird (though something pitch-framing something).
But the obvious difference is that the vast majority of these contracts were given to older players. The only one which is even slightly comparable in age, is the Upton deal, which covered his age 22-26 seasons, and which looks, with one season left to play, to be on the positive side. With Tomás under contract for his age 24-29 seasons, I'm hopeful that Tomás will be along those lines, or at least immune to the sharp drop-off we saw in some of the above cases, where the aging curve was more of an aging plummet.
2015 Payroll: going up!
We have no details of how the numbers break down, but even before this, the arrival of Jeremy Hellickson had the D-backs at a payroll of round about $97 million. Adding a ballpark figure (hohoho) of $9m for Tomás on to that, and you're looking at $106 million. That's still less than it was on Opening Day this year, but I do think this probably rules Arizona out of the running for James Shields, whose price would send the total soaring to somewhere past $120 million. With limited scope for freeing up salary - Montero is still about the only large contract we have much hope of shifting - I think a trade is now the most likely way of addressing our obvious need for a good starting pitcher.
Cliff Pennington might do well to join Mark Trumbo in selling any local residences, as between them, they will probably cost around $9 million in 2015. Trading them for a cheaper - pre-arbitration or 1st-year arb - pitcher seems like a way of improving the team which simultaneously cutting costs for next year; after that, the new local TV contract should permit the payroll to increase, so it becomes less of an issue. Increasing revenues may help explain the sudden willingness of the front-office to spend, hoping to secure talent before the rising tide of national income, floats all free-agent boats higher.