it's easy to figure out. electric companies charge by the kilowatt hour. a 400 watt lamp uses .4 kilowatts per hour. on your last electric bill, there should be a breakdown of how much the electric company charges you per kilowatt hour. let's just say they charge you $.10 per kilowatt hour.
the math would be .4*18= 7.2 kilowatt hours per day. multiply this by an average of 30 days = 216 kilowatt hours per month. multiply that by the amount charged per kilowatt hour. 216*.10= $21.60 per month to run your light.