If your heater draws 600 watts, that calculates to about 5 amps. So use no less that a 14 ga. extenstion cord.
Cost? (based on rate at 1000 kw use per month)
In Omaha Nebraska, you are looking at $. 06486 a kilowatt, operating your heater for 12 hours would cost you about $. 47 per 12 hour period.
Here in Grand Island, we have the second lowest priced electric power in the nation, our power costs are . 046 per kw, per 12 hr period, it costs me about $. 33 .
IN New York City, the highest priced electricity in the nation when it's on, power is $. 15677 per kw, and per 12 hr period it would cost you $1. 128 .
To find out how much you pay, take your electric bill, take the price you pay, divide it by the amount of kilowatts you used, you then get the price per kw. Take that figure, multiply it times 12, then by . 6 . There's your answer per 12 hour period.
I plug mine all the time when it's cold, even at work. I work at the power plant, and get all the free power I can take home in my lunch box

. It's nice to get into a warm truck.
Or look up your rate at
http://www.les.com/pdf/2003KPMGSurvey.pdf