The AVERAGE US electricity rate is around 14 cents per 1000watts for 1 hour, or another way to put it is 14 cents per KWH ( Kilo Watt Hour). So if you run your light on all the time for a month it will cost you 28 days x 24 hours = 672 hours x 1000 watts= 672 KWH. 672 x 14 cents = 9, 408 cents = $94.08. Input your actual rate in place of the 14 cents and you too can figure out how much that light is costin ya.
If you lived where I did and if you run your light only at off peak hours ( As I do) you may also be blessed with 3 cents per KWH rates. You have to ask the utility company to rate your electrical useage that way to do it sometimes ( I Did).