I think the math goes something like this:
1000 watts == 1 Kilowatt (KW)
1 KW used for an hour = 1 Kilowatt Hour (KWH)
1 KWH per hour over 12 hours = 12 KWH per day (Let's say your are in 12/12 flowering schedule)
12 KWH per day over 30 days = 360 KWH per month!
Your bill will show how much each KWH costs in your area. For example, in my neighborhood each KWH costs about $0.25 including all the separate fees.
So $0.25 times 360 KWH used in a month of 12/12 lighting is $0.25 times 360 = $90.
An HPS bulb will likely use the wattage on the label so 1000 watts actually means 1000 watts used so just plug in your local $ per KWH and you're done. For other wattage bulbs, begin by dividing their actual power draw in watts by 1000 to determine their KWH rating. So a 600 watt bulb = 600/1000 = 0.6 KW per hour.
Other light types actual power usage can be more complicated to determine. I use a bunch of big CFL bulbs that claim the provide 150 watts of light, but they only draw 40 watts from the outlet. So each of those puppies cost me $3.60 for a month of flowering. Not bad except I use a bunch of them sometimes which causes some "friction" with the saintly spouse.
Regards,
-Grouchy