depends where you are. in California, price is about 150 a month id say. Also, i wouldn't expect a pound from a 400w, or even a 600w, especially since this is a first timer grow. a half pound is more realistic id say.
take the wattage on the light you plan to use and divide it by 1000 to get a kw from w. then multiply that by the cost of your electricity and then multiply it by how many hours a day you'll have it on.
ex: you have a 1000W light, your electric is 0.06 cents/kwh, and you run the light 18hours a day
1000W/1000=1> 1kwx0.06cents/kwhx18hours=$1.08 a day to run the light.
multiply that by 30days a month and you get how much it would cost you per month to run the light.
hope that helps. if i made a mistake anywhere someone please correct me