Cost of running a 1500 watt electric heater will depend on your electric rates and what you pay for each KWH.
For example my kwh cost is 10 cents. So over a 2 hour time period the 1500 watt heater running nonstop would use 3000 watts.
So the 1500 watt space heater would cost me 30 cents per every 2 hours the heater runs.
If the electric heater is run for 16 hours it would use 24000 watts and would cost me $2.40 cents a day for 16 hours a day.
$2.40 per day x 30 days would cost $72.00 per month.