As others have posted, it depends on your electric and service rate, taxes, etc. Also how long you run your lights for; 12/12, 20/4, 24/0, etc. Here we pay 9.8 cents a KwHr, and another 2.3 cents per KwHr for distribution, service fees and taxes. So its basically 12 cents a Kwhr. Some pay as high as 24 cents. In California its a sliding scale; the more you use, the more you pay per KwHr! At least with PG&E that was the case when I lived there.
Some examples, using 12 cents per KwHr as a low and 24 cents as a high, low/high in $USD, using a 30 day month:
32W T-8 pair on 20/4, 1.28 KW/day, $0.15/0.30 per day, $4.50/9.00 a month
32W T-8 pair on 24/0, 1.54 KW/day, $0.18/0.36 per day, $5.40/10.80 a month
100W floro on 20/4, 2.0 KW/day, $0.24/0.48 per day, $7.20/14.40 a month
100W floro on 24/0, 2.4 KW/day, $0.29/0.58 per day, $8.64/17.28 a month
250W floro on 12/12, 3 KW/day, $0.36/0.72 per day, $10.80/21.60 a month
250W floro on 20/4, 5.0 KW/day, $0.60/1.20 per day, $18.00/36.00 a month
250W floro on 24/0, 6.0 KW/day, $0.72/1.44 per day, $21.60/43.20 a month
400W HID on 12/12, 4.8 KW/day, $0.58/1.16 per day, $17.40/34.80 per month
400W HID on 20/4, 8.0 KW/day, $0.96/1.92 per day, $28.80/57.60 a month
400W HID on 24/0, 9.6 KW/day, $1.15/2.30 per day, $34.56/69.12 a month
1000W HID on 12/12, 12 KW/day, $1.44/2.88 per day, $43.20/86.40 per month
1000W HID on 20/4, 20 KW/day, $2.40/4.80 per day, $72/144 per month (yes, spendy!)
1000W HID on 24/0, 24 KW/day, $2.88/5.76 per day, $86/173 per month (yes, even more spendy!)