I use an oil filled radiator heater. it has a low setting that only draws 700w where as the high setting draws 1500w either way even if its on constantly it will probably only run about an extra $20-30 dollars a month. just offer to pay the difference if you can or offer to work it off in household work. honostly you only need to heat it up about 15 degrees so I would say that shouldnt take too much. also do whatever you can to insulate the basement from the cool air outside
Cool does your oil have a temperature gauge shutoff to wear it reaches a certain heat and shuts off if so what is your model thanks