Cousins is a risky acquisition because he's such a knucklehead. Highly emotional, bordering on childish at times. That being said, he's uber talented, and if you're the Celtics, he's a critical piece to speeding up a rebuild.
If Cousins is traded, he won't come cheap. I can't imagine the Kings trading him to begin with, but if they do, there will be tons of suitors, and he won't come cheap. The one staple in NBA trades like these, is that the team trading the best player rarely ever gets equal value in return. The Kevin Love deal may prove to be the exception to the rule, via Wiggins, but most of the time the best player is sold for 50-80 cents on the dollar. It's just how it works in this league. So while we'd expect the Kings to say no to all these possible deals, understand that while they would prefer to say no, if Cousins has to be moved, they will eventually be forced to say yes to the best offer on the table. None of those offers will likely contain a franchise player going back to the Kings. If a team had a franchise player to trade for Cousins, they either a) wouldn't trade him, or b) would be defeating the purpose of adding Cousins.