How do you calculate if it's worth it to insulate?
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&amp;State=NY&amp;RE=1&amp;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.