You've gotta look at an electric bill and figure out what you pay per kwh or call the electric company. You can safely assume a 400 watt light consumes approx. .4 kw per hour of operation. So for example if you run 24 hours you use 9.6 kwh per day. My electricity can run around .10 us per kwh so roughly .96 cents per day x a month (30 days)= $28.80 a month (some reason seems I'm coming up high). But anyways you can see the thought process just plug in your own information to match and change hours according to schedule.
