I am in Michigan, and last year our gas bills got up to $400 a month (2600 sq ft house, but average height rooms, no cathedral ceilings, etc.). We didn't want that again, so in the spring we added a lot of insulation, up to the R68 level (well above builders code level of I believe of R46). So far, our gas bill for comparing Oct/Nov this year and last year is half. So we are very happy in that investment.
I believe we had rolled insulation up to R13 or 16 (the level up to the rafters, that was code back in 1963 when the house was built I think), and then they blew in more to reach R68.
Also, by adding the insulation, and also an attic fan, we didn't even turn on our AC this summer. But that may be because it wasn't a very warm summer this year.
Hope that helps!