Electricity cost per hour
1500w / 1000w = 1.5 kwh. Every hour you run (assuming it is at 100% peak power) you use 1.5 kw. Just multiply that rate by how much it your power company charges per kwh.
Estimating high at 10 cents per kwh it would be $1 for every 10 hours you run the heater, or $2.40 per day, or about $72 per month if you ran it 24/7 for 30 days.
It is FAR cheaper to use these types heaters to heat one small room than an entire apartment or house if you are only in one room the majority of the time.