You'd have to save more than or less than $318/year depending on what assumptions you make on the price of borrowed money vs. the after-tax return you might have expected on it otherwise, as well as the current price & inflation rate of fuel pricing There are several net-present-value calculators online that can put it in perspective.
Even so, using a very crude insulation upgrade savings calculator, it's hard to say that you'll go NPV+ in 10 years with most reasonable assumptions:
http://www.builditsolar.com/References/Calculators/InsulUpgrd/InsulUpgrade.htmSouthern NH is about a 7000HDD climate, insert your current fuel price and 90% efficiency on the boiler for annual and 10 year (simple, not present-value) savings.
A more cost-effective way to go high-R in that climate would be to do R30 (4.5-5") of foam at the roof deck, and another R30 of wet-spray cellulose under that for a net R60. Closed cell foam is a premium product at a premium price- great for when you need/want the vapor retardency or high R-per-inch, but it's an expensive way to super-insulate. If your splitting the R value between cc foam & fiber with the foam on the cold side (exterior) of the fiber, be sure that the foam is at least 50% of the total R in your are (40% is OK if you're in the Portsmouth coastal region).
If it's basically a dead-space/storage-space attic, it's fine to use dry-blown cellulose the attic floor for the fiber, but be sure to air-seal the attic from conditioned space well. A 2x10 joist filled to the top will yield about R30-R33 in the center between joists, R25-ish overall (due to the thermal short-circuits of the joist. It'll make the attic space colder, but won't get/stay below the dew point. (You can monitor it for awhile- if the RH stays above 80% all winter you could put in a small dehumidifier to guarantee it never condenses.)
Natural gas, or propane? (There's a ~300%+ difference in $/BTU between the two in central MA right now, but NG prices are highly variable between utility companies in New England.)