We live just north of NYC, and the house was built in 1906. We're putting on a new roof and trying to figure out if it's worth it to insulate too. It's a cathedral ceiling on the inside, and we do not have vented soffits right now. The contractor says it might not be worth it since he'd have to vent the soffits etc.
Insulating the roof will be worth it - you will save significantly on heating and cooling, and doing it while the roof is open is the best time. There are other things you can do at the same time to maximize the return on the roof insulation. Reducing the chimney effect from basement to roof-line with proper air sealing will multiply the savings.
One caveat: if your roofing contractor does not know if it's worth it, he's the wrong person to be installing insulation. The right insulation, installed properly, is worth it. The wrong insulation installed incorrectly is not worth it, for example, it's easy to make R-19 fiberglass batt insulation insulate as poorly as an R-2. As a rule of thumb, if his prior experience says that customers aren't getting a good payback on insulation, then he or his subcontractor is not insulating properly.
You don't want to just "insulate" - you want to get an energy audit and do the kind of insulation and air sealing that's needed for your house in your climate.
There are tons of incentive programs in NY state (http://www.dsireusa.org/library/includes/map2.cfm?CurrentPageID=1&State=NY&RE=1&EE=1), as well as special low-interest loans run through the utility companies that you pay via your utility bill - and whose payments are designed not to exceed the amount saved by the efficiency measures you make.
There are also federal tax rebates for energy efficiency measures. Find a BPI or RESNET HERS certified auditor to assess your house and help you figure out how to maximize your savings. Between rebates and energy savings, you'll find that insulating is one of the smartest investments you can make for your family.
It's not very clear what the assumptions of the model, but I found this tool that makes recommendations based on heating systems, zip code, etc. Taking those factors into account it recommends R38. Just based on zip code it is recommending R49.
I also learned that you can use dense pack cellulose without having to vent the space since humidity is not a problem because there is no air flow to carry the humidity, so that should reduce the cost.
For somewhere that has cold winters and humid warm summers your should be looking to insulate. The cost to heating and cooling a house is greatly effected by how you loose all that heat.
Although specifically for your dwelling its not possible for me to say, However on average over multiple dwelling analysis with infra-red heat loss it has been shown that you loose 26% through the roof, 33% through the walls, 18% through the windows, draughts 12%, floors 8% and doors 3%
The costs of this add up significantly especially through winter times or in climates that are colder. You can also shove the whole environmental thing in there as well as your going to be using more energy to heat it however most are not to interested in environmental effects.
Using insulating foam board followed by fibreglass wool in both the walls and roof along with double or triple glazed windows will dramatically decrease your energy consumption.