An oil filled electric space heater has a few different settings and I believe the lowest setting is 700 Watts while the highest setting is 1500 watts.
If you have the electric oil filled radiator space heater on the full setting of 1500 watts then assuming your electric rates are 10 cents per kwh it would cost around 15 cents per hour to operate the electric oil filled radiator space heater.
If you run the space heater for 8 hours per day your cost would be $1.20 per day per space heater you run.
If you run the space heater longer or you add more space heaters then your cost would go up.
But the space heater will stay on until it reaches the desired temperature and will then shut off again.
So the radiator space heater won't be heating all those 8 hours.
The cost to run the space heater per day would go up depending on your electric rates.