I know they aren't struggling for money, but if by their calculations trading Pryzbilla away allows them to sign an extra great/good player in the free-agent market, they might be inclined to do it. This is not really about saving cash, but becoming even bigger players in free-agency. Those are the things they have to weight. Again, not saying it would be the correct choice, but certainly some considerations should be given... especially since Pryz's playing time has been on a downward trend.
And as I said, other things factor in it like their willingness to trade some guards and forwards that they apparently want to move. So you might take a blow in this particular trade, but improve your chances of gaining good players elsewhere... that's what they have to consider, and I'm no position to evaluate their situation and opportunities. You know, do one of those Ainge trades that look bad at the moment, but payoff quite well in the future?
If I would trade Perk for Scal and Tony? No sense in answering this since teams circumstances are different.