I would not trade Lee. While he had a bad season, I think he's capable of bouncing back and that contract is worth every penny if we get the real Courtney Lee.
What is there for Lee to bounce back from? When you control for minutes per game, most of the stats aren't that different. His 3p% went down, but it was compensated by his 2p% going up.
I'd argue that if both are Celtics next season, Bass is more likely to out-perform his 2012-2013 effort than Lee is.
When Shane Battier was traded to Memphis at the deadline in 2011 as a rental, he was traded for a draft bust in Hasheem Thabeet and a future first (later traded to Minnesota, who has it at #26 this season).
If Bass or Terry is traded at the deadline, that's probably the max value that you should reasonably expect. Maybe a worse draft pick if you expect to get a young player better than Thabeet. I'd be willing to dump Terry for that. I think I would be tempted to hold onto Bass until I've kicked the tires on a few trade scenarios (including sign-and-trades) for this summer and the next. I'd hold off on blowing things up until I have tried that.