If you're using the new Led light bulbs then it costs not even a half penny to leave your light on for an hour.
I have light bulbs that are led bulbs and they use 9 watts of electricity per hour and I leave them on for 8 hours per day most days.
So in 8 hours that 9 watt light bulb would use 72 watts of electricity in one day and if we take that 72 and times it by approximately 30 days in a month the led light bulb would use 2,160 watts of electricity per month.
Electricity costs me 10 cents per KWH so leaving that light bulb on for a month for 8 hours per day would cost me around $0.22 which is very little money.