Are you on a basic plan, a time of use plan? Is the ballast set up to run under 110V or 220 V? Under the SRP basic plan, with a 110v ballast, you will be paying around 36.28 dollars every 30 days at 18/6. You can throw all that math out the window if you are using a different plan or a 220v Ballast (half the price to run it under a 220V ballast)
The real killer here is , how much air are you exhausting out? If you are exhausting 200 - 400 CFM of air out of your house, in turn your central a/c will have to work extra to cool down that extra influx of air in your house. An 1800 Sq. Ft House will have all the air in the house replaced one or two times per hour depending on the size of your exhaust fan, leading to your central a/c cooling down the replaced air and working longer.