Perk turned down a deal that was between $22 and $23.4 million, which is less than $6 million per season. That's the most the Celtics can offer him for an extension right now, but that changes after the season. I wouldn't read anything into the rejection, since Perk has repeatedly indicated he wants to be back.
I don't think we can afford to move Perk, since he's our only healthy center. Moving him basically ends our season (and most likely, next season, too). Philly wouldn't move Turner for a bench guy (who will be a free agent), but even if they would, I don't think we can afford to sacrifice size.
The Celtics are an old team, but they're contenders now, and probably will be next year. I don't think it makes sense to get worse in the short term just to start the rebuilding process sooner.