It depends on your electric rates. But electric space heaters use a lot of electricity.
On High they use 1,500 watts each.
Assuming your electric rates were around 10 cents per kwh you could expect to pay around $1.50 per 8 hours of use per space heater.
That's not including the electricity transmission costs and other taxes.
If you use the space heater for 8 hours a day you should be looking at around a $50.00 per month increase in your electric bill.
It might be cheaper to just turn up your furnace.