A realistic big man target would be whatever minimum salary guy you think can be acquired for Barbosa if he becomes expendable due to a healthy Bradley.
The math just makes it seem unlikely that the Celtics can acquire a quality big making a significant salary. The thing that would change that is a player making more than the minimum being injured so severely that he is effectively trade ballast only being included to make salaries match up.
Why wouldn't the math work out?
Bass makes 6 mil. Terry and Lee are both making 5 mil each. Can't we take back 150% salary?
The Celtics are also facing a hard cap because the team used the full MLE and are close enough to it that they are probably only doing a trade where they have less salary coming in than going out.
If I were going to be realistic about suggesting trades, I would work under the assumption that Ainge isn't trading Rondo, Garnett, Pierce, Bass, Terry, Green, or Bradley unless he is being offered a ridiculously lopsided deal. The exception is if the team is doing so poorly or is so injured that Ainge is in blow-it-up or hail-mary-trade-to-salvage-the-season mode or if a player is a locker room problem who the team is willing to practically give away.
I could see Lee getting way fewer minutes than he expected and becoming unhappy enough to embrace a trade, so I'd look for someone who has around the same salary or a bit less, perhaps an under-performing former lottery pick who has failed to crack his team's starting lineup but who you think would do better in the Celtics system. I'd abandon the idea of acquiring a veteran with a track record of being an above average starter. If that's what you want, you are better off hoping that a player with an expiring contract is bought out and wants to be a Celtic.