The price of such new gear is much more difficult than older stuff that gets bought and sold over and over again. While the price of a P45+ is pretty stable, it seems like IQ260 can go for anything. I'd guess that patience is one factor, if you need to sell fast then the right buyer may not be there for you in time and you need to drop the price more than you otherwise should.
I don't think other products like CFV-50c, 135 cameras or the Pentax affects it that much, the market might have changed a bit, but Phase One still stands strong as the prestige brand of high end digital backs and the IQ260 still has a number of unique points and with some patience I think the right buyer will come. It's fullframe, it has long exposure. I have not so good idea of the right price though, although in a private sale it should be considerably lower than a dealer pre-owned sale. Maybe $20-22k?