Check you electric bill and see how much your power company is charging per kWh, it's probably $0.10. In that case you're looking at about $35 per month if you run the light 18hrs a day, and about $25 when you run the light 12hrs a day.
Remember there's also delivery charges etc. If you look online your utilities company probably has a cost calculator
http://www.rapidtables.com/calc/electric/watt-to-kwh-calculator.htm use this site to calculate watts to kwh then multiply outcome by your rate.