There are two ways to approach getting two matching sides to a trade (actually probalby more than two but two relevant to Cousins I think).
One is to match up talent with talent within the financial constraints of salary caps. In this case, Cousins only makes $3.8M so it is hard to match that up. The Sullinger, Bradley, Melo package probably doesn't do it (too much potential vs. established value) so you need to look at the other way of matching up.
The other way is to take back overpaid junk. In this case that could be Salmons or maybe Outlaw and several others. In order for this to work for the Celtics, I think they need to expand it. Something like:
Green, Lee, Bradley, Sullinger (or Melo?)
Cousins, Evans, Salmons
In order for this to work, they need to believe that Green is worth something and that Evans is leaving anyway. It is kind of
Green + Lee = Cousins (they are saying they want more established vets)
Bradley + Sullinger = Evans (this is really good value for them)
And we take Salmons to save them money and we would need someone to replace Greeen.
There are many other possible combinations but I think this is the type of deal that would be needed for it to work.