The math on this is more correct as opposed to the former poster who did not consider power consumption.
600W X 18 hours = 10800WH Then we can Convert to KWH: 10800WH/1000 = 10.8KWH (this is how much KWH per day the 600 watt will draw) We're running it for a month: 1 month = 30 days (or 31 days) 10.8KWH X 30 = 324KHW / month. Now we calculate cost using the rate of 23.5 cents / KWH. 324 KWH X .$235 = $76.14
Come on people let's educate not just do the work for people.