The way I see it, even if we did trade for Cousins, we would ultimately have to sign him to an extension around the 6/200M range as well.
So I figure it's not horrible news that the Kings would extend Cousins this summer, because a few more bad seasons and he could still be on the trade market.
That being said, I doubt Ainge wants him. I mean, you hear Ainge in rumors about Butler, and guys like Noel, but have you heard him in Cousins rumors? Rarely, and the only time he touches upon Cousins he usually gives negative feedback, like "we don't need locker room cancers" or "from a distance, looks like Cousins has a lot of problems...".