I did the math a couple of days ago and the max they could offer was a bit above $36 millions, including the raise this season. I mentioned the exact number in some other thread.
Fair deal for both parts. Perkins could get a little more in the open market, but considering the CBA negotiations and the risk of more restrictive rules and less money for salaries, he was smart to cash in. The Thunder are set for the future. They need a scoring big off the bench (ideally a guy with range) and to work Harden into the starting 5.