A year ago (August 11, 2008), I did some math to see how much my tank costs me in electricity. Not the most fun thing to know, but still... here's the updated version, 12 months later. Some equipment has been changed out, and the rate of electricity is 10.3¢ now instead of 12.6¢:
 

Basic operational cost of my tank in electricty consumption is $64.73 per month.

If the heaters only cycle on once a day, it costs me $6.80 per month. If they come on twice because they are fighting the window A/C unit, it costs $13.60 per month. I'm trying to avoid this by bumping up the A/C temperature to 75F at night so the heaters don't come on. It requires that I remember to drop it back down to 73F for the day period though, which keeps the tank from rising above 81F.

The window A/C is used all summer long. So during these hotter months, it costs about $1 per day to cool the fish room, or $31.26 per month. I used the Kill-o-watt to determine that the compressor only runs 12 hours out of every 24 hour period.

The vent fan runs 24 hours a day, which costs $5.71 per month or $68.52 per year.