If you dont't have a cheap heat source, heated greenhouse must be made of insulating material.
I bought a plastic greenhouse last year for starting seedlings, luckily I started with lettuce and onions, because when temps were just bellow freezing, soil in pots was solid frozen. I tried with electric heater only to find out it must be constantly turned on.
But day temps inside were at least 5C/10F higher than outside even on cloudy days, so lettuce, chard, parsley and onions grew realy well. No chance tomatoes and peppers could survive nights
.