you should be able to get an approximate cost by getting the cost/kwh from your utility bill and then determine how much electricity the heater uses (alot!) and how long you think you'll use it.
IOW
total cost = heater electricity consumption(1000 watts, 1500 watts, etc) TIMES electricity cost (per KWH) TIMES the number of hours in use.
Example:
a heater that uses 1500 watts of electricity uses 1.5 KWH in one hour of continuous use. If your electricity costs 10 cents per KWH and you run it for 5 hours a night for 30 days, the cost will be
1.5 X .10 X 5 X 30 = $22.50