There are some variables here, like the temperature that you keep your tanks and the temperature that your house is kept at. A warm house and a cool tank will minimize your costs. Being in southern California, this might be the case for you.
I bought a Kill-A-Watt meter a couple of months ago and did some testing on my 50 gallon. I keep my tanks on the cool side, at around 75 degrees. Room temperature was normal winter temps for me - 67 degrees during the day and 65 at night. Electricity cost is .10 per kwh.
Heater: 2*100W = 36 kwh per month = $3.60
Light: 2x32w T5 NO for 8 hours = 15.36 kwh per month = $1.54
Power Head: 8W = 5.76 kwh per month = $.62
Total: 57.12 kwh = $5.71 per month at .10 per kwh
Interestingly, the heater ran for exactly 25% of the time, the figure mentioned by HD. And no, I didn't forget the filter, I don't use filters on my tanks just a powerhead for circulation.
Life isn't stone, it's water.
PSA: When talking about baby fish, "fry" is plural.
I have a fry. I have some fry.