I did the math a month or two ago on electrical usage with a cabinet incubator. We used a killowatt meter and kept it hooked the entire 21day period. With our electrical rates we figured the total electrical cost to be around $5 per hatch. Without going back and looking at the numbers, I dont remember how much electricity was used or my cost perkw of electricity. With a 298egg incubator this comes out to about $0.016 per egg.
Your incubator size and electricity cost could make these cost vary somewhat, but even with a LG, you could probably figure 2cents+/- per egg for the electricity if the incubator is filled to capacity.