((watts_of_bulb) * (hours_per_day_bulb_runs) * (days_in_month) / 1000) * cost_per_kwh = estimated_cost_per_month.

So, say it's a 1k light, and you pay $0.20 per killowatt hour (info usually on electric bill), just plug in the numbers:

((1000) * 12) / 1000 * 30) * .20

is

12 * 30 * .20

is

360 * .20

is

72 (dollars)

And so on.