Cheaper possible but usually a reason.
Not sure if I agree old chap. The market is simply flat, not much moving, albeit a tad better than 2009/10. If there is an owner fed up and wants to sell, he has to go via the price. Too much offers around.
The TO wants an airplane 10 years or younger, the Sierra mods are very interesting, but no 1, 2 or S2 is young enough to make it a 10 year old aeroplane, unless you think a Sierra mod males it a new airplane. (which it essentially does, but...) and I doubt that they will take on the costs for a Bravo mod. Question is, how much would they be able to gain speed and range wise compared to the newer Pratts of the Bravo. But a call won´t hurt I guess...
They offer something, but no engines:
CITATION 550 BRAVO UPGRADES