The approximation for electrical use in heating is using heaters that can output roughly 10 watts per square foot of your house. 12-13 if your house is poorly insulated.

At full power, that means that a 1,600 sqft house requires 16 kW of electricity. At a cost of 13¢ per kWH, that's about $2.05 for that 16kW of electricity. Of course, no residential house will ever realistically have 16kW available to it, nor would heating ever require a 100% duty cycle. But this is for comparative purposes.

The equivalent amount of gas power would require about 54,000 BTUs from a gas furnace (about 3.4 BTUs per watt). One cubic foot of natural gas provides 1,032 BTUs which means that generating 54,000 BTUs of heat requires about 52 cubic feet of gas (we'll round that up to 65 to account for inefficiencies of a furnace; top end furnaces are in the 80-80% efficiency range). 100 cubic feet of natural gas costs about $0.48, so the cost for natural gas would be around 30¢

Again, neither will (nor could, nor should) run constantly, but it's a sobering comparison.

EDIT: Comparisons were hours vs. seconds; fixed.