The cost of running it will depend on the temperature setting of the chiller and the temperature setting of the room. In normal weather, my chiller was set to go on at 79 and off at 77. The ambient temperature in that room is 78. The chiller generally ran about one hour on for every four hours off. The house is air conditioned.
Now that we have had temperatures in the 90s for the past week, and have had to raise the temperature of the air conditioner during the day to cut back on excess cooling, the chiller runs about 1 hour on and 2 hours off. I changed the chiller setting to 78 and 80, but that didn't change the interval. My tank is a 75 gallon with VHO lighting.
Hopefully you can translate that into the answer to your question.