Probably a lot more than just eliminating drafts from the coop area. Many folks do not add heat for winter chickens, unless your closer to the arctic circle.
Look at the kilowatts used at your house, on your bill. Divide those used kilowatts into the dollar amount of your bill. You'll likely get $.18, give or take, depending on your electric company. Each watt used over an hour, equals 18 cents. Here, I've computed the cost of running a 250 watt bulb at $25 a month. Running two bulbs costs $50 a month.
Simply using the cost stated per kilowatt hour advertised by your electric company is a bit fake, because there are so many stinkin' added fees and surcharges.