The full story is more complicated of course. Since the weather is never exactly the same month to month, I'll not worry that we've only one gas/electricity bill has come since the install and that bill had 3 weeks prior and one week post. That bill was 240, and the previous (including a week of visitors) was 260.
Let's say we save $50 per month for 6 months out of the year (4 cold and 2 hot). That makes $300 per year.
Our total cost was about $5500. Here's where I need a finance expert to tell me the pay back time frame (including interest, expected cost increases of gas/elec, value added to the house, etc.). In any case, I imagine that more than 10 years will pass before it is 'paid for'.
I'm aware of a number of cases in which improved efficiency leads to greater resource consumption. On a related note, I have kept our thermostat at the same temperature as before. The stat is located in a narrow hallway right next to the closet containing the furnace and water heater. I suspect that the temperature in the rest of the house is around a degree or 2 warmer than previously. I would probably lower the setting to make up for this, but the womenfolk would likely protest.
Mercifully, we have a warming trend this week. Now, we're off to lunch in our car. I'll crank the heater as needed.