Since electric costs vary from city to city It would be best to contach local electric co & ask them.....

You have so many variables such as ambient temperature and how warm are setting the soil thermostat I doubt any one could guess very acurate. I maintain a bottom heating plate 3 feet by 6 feet insulated from below on a bench in my greenhouse. The cable is 60 feet and 300 watts. the temperature is set at 70 degrees. I leave it on 365 days per year 24 hours per day and do not think the cost is excessive. Al