We were scared to death, when we saw an ESTIMATED gas bill last month (OLD gas hot water tank, and NEW gas furnace...). Everyone around here (1100 sq ft starter ranch-style homes) was complaining about $250-300 gas bill. Ours was $100. This month's bill arrived yesterday, for even colder temps (and DH turned the thermostat UP this month, to 69 or 70, AND it was an ACTUAL reading. Well, it was only $139!!
Then I talked to SIL, who often has $300 gas bill. Turns out, her electric and gas are COMBINED. Didn't know that existed. Makes more sense now, though...our electric adds another $100-125 (so total,$200-$260/month in winter).
Don't know which way is more common, combined or not...but we are LOVING our new furnace (but will love it BETTER when it's paid off, lol).