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.