ESPN has the details, via Ric Bucher.

Perkins will receive almost $36 million fully guaranteed over the course of the four-year contract, his agent, Bob Myers, told ESPN The Magazine’s Ric Bucher.

As part of the deadline deal that sent Perkins to Oklahoma City, the Thunder trimmed a tiny bit of cap space (a little more than $1 million), which gave them just enough additional wiggle room to help hammer out a contract extension. With Boston over the salary cap, the team couldn’t offer more than $22 million over four years, while Oklahoma City was able to use that sliver of cap space to offer Perkins as much as $13 million more on a four-year deal.

So the Thunder signed Perkins for $9 million a season, which is about the going rate for a starting center. Perkins is widely regarded as one of the best defensive centers in the league, and on-court/off-court numbers at 82games support that. OKC obviously believes he will be good addition to their core of Kevin Durant and Russell Westbrook. One knock on the Thunder is that they aren’t tough enough, and the seasoned Perkins will definitely help in that area.

I highlighted the bit about Boston because it’s a little misleading the way it’s written. It’s not that the Celtics couldn’t offer Perkins a bigger deal, they could, they just elected not to. With a soft cap, a NBA team can re-sign its own players for whatever the two sides can agree on. The Celtics made a financial decision to trade Perkins away because they knew they weren’t going to pay him when his deal was up after the season.