I'm going to predict that (assuming Cousins is made available) it would take an absolute minimum of Sully, Smart and #16 as the starting point for any negotiations.
I would be very, very surprised if the Kings would consider any deal that didn't include Smart.
I think that in order to actually convince them to pull the trigger, we'd need to sacrifice almost every <25 year old player we have with legit upside. That probably means something like Zeller, Sully, Smart, Young, 2015 Celtics pick (#16) and 2016 Brooklyn pick.
Then they MIGHT listen.
Demarcus Cousins is a franchise player - centers who put up statlines of 24/13/3/1/1 come along once or twice in a generation. The most recent one prior to Cousins was probably Tim Duncan (if you consider him a center) or Shaq.
If you are giving up a player of that talent level, and you aren't getting at least another all-star in return, then you're going to want to take half the team with you.