Find the total watts:
8*23w = 184 watts total or .184 kilowatts
Figure out you kWh price, around 10-20 cents, typically. It's on the bill, usually.
There are 720 hours in a 30 day month(24 hr*30 days). 18 hours * 30 days = 540 hours
So, multiply the total number of hours, by the total...