The absolute max my tank could cost per month is about $120/month. depending on unit price of course. That's all the pumps on, all the lights on and the heater on full 24 hours/day all month. Of course, lights are only on about 12 hours, heater is on maybe a total of a couple hours/day max.
Edit: Just checked our rate, 19 cents/kwh so that makes it an absolute max of $77. More realistically I'd say it's in the $20/month range.