If the UTHs are the ones I'm thinking of, where the small one takes 4 watts and the larger one takes 8, and you're running the daylight bulb for 12 hours a day and the night time bulb for 12 hours a night, I'm coming up with a total of 80.64 kilowatt-hours per month.

If you have more accurate information regarding your exact usage, that would help. But take a look at your power bill, and see what they charge you per kilowatt. In my area, due to my lower power usage, they charge me about 9.5 cents per kilowatt-hour. At that rate, your setup would cost me $7.66 per month. BUT, energy rates vary widely depending on where you're at. So take a look at your bill to see what your rate is.