First of all, you can pretty much take initial damage estimates and double them. Second, Sandy was a hurricane until just before landfall. I watched the sat photos and microwave passes, it developed a distinct eye Monday. And that in turn, IMO, changed the nature of the storm when it hit land. It was expected to be transitioned to an extratropical storm a day before landfall, which means the higher winds aloft would be brought down to the ground and the storm would be strengthened by baroclinic processes, and we would have seen a lot more power outages further inland. I don't think that happened much, the landfall progression of winds were much more typical of a tropical system and by the time the worst of the storm reached where I am, in NW Montgomery County in PA, the winds were not that bad. And third, the damage was very different between Irene and Sandy. Irene was mainly an inland flooding event. Sandy was a coastal flooding event - because it apporoached the coast head-on instead of with a glancing blow. The real wildcard for damage estimates is going to be how much repair the subway system will need because of saltwater immersion. They just don't know, but I saw a report that estimated it would take $55 billion to fix the system if it flooded. Now, that would probably be the entire system, but I think it's a good change that the subway system damage will add a fair chunk to the final damage bill.
So in a way, they got part of the forecast wrong. Baroclinic factors did not juice up the storm the amount they envisioned. But it was a tropical system and a hurricane up until just before landfall.
Agree with all that, but Sandy had no back side when it hit land. Was having trouble wrapping up its west end which was being stripped away and taken north by the cold front. Then when it sped up just before landfall, it could not pull anything from the west side at all. There was no moisture to speak of on its SE side when landing. My guess is when the cold low pressure joined up with Sandy, it pulled Sandy in and sped up its forward motion basically ripping the Hurricane/Tropical Storm apart. So the joining of the storms actually made Sandy less powerful, which was the opposite of what they predicted. Forecasters forgot that cold air is more dense then warm air.