He is a contributing player on a playoff team. To get him you have to give the Thunder something they need and they certainly don't need a late 1st or 2nd round picks given their treasure trove of picks from the George and Westbrook trades. Now getting them out of the luxury tax probably has some value as would giving them other solid contributors
I think something like this makes at least some level of sense for both teams
Noel, Roberson, Muscala for Smart, Green (and then Boston cuts Poirier or finds another team to take him). I think the Thunder might rather trade Ferguson than Muscala, but I like Muscala as a stretch bench big. Shaves about 500k from the Thunder so they get very close. With Ferguson they get under the tax line, so maybe if they want that, Boston can pick up a 2nd round pick or two (Boston doesn't have many of those and they are useful in trades).
It is a risky trade for Boston, and losing Smart would hurt, but I do think the trade has a chance to make Boston better this year (with more balance and a much deeper bench), and if Noel works out, could be better long term as well. Roberson can be the bench defensive wing or can be moved in a separate trade since he is an expiring contract.