Average where I am is 6 cents for the next couple months, 7 cents for the summer.
1000 watt lamp would cost this much if you pay 7 cents a KWH.
24/0 = 24kwh/day*30 days = 720kwh * $.07 = $50.4 dollars a month
18/6 = 18kwh/day*30 days = 540kwh * $.07 = $37.8 dollars a month
12/12 = 12kwh/day*30days = 360kwh * $.07 = $25.2 dollars a month
So if you used your CFL's for veg, and the 1000 for flowering, you would increase your energy bill by less than 25 dollars. It depends on how many CFL's you are using. Of course, its a little ridiculous to be using a 1000w HPS on just a couple plants, if that is what you are doing, so maybe consider a 600w HPS, which is much more efficient anyways.