If you can close the door and lessen the heating load on your furnace, it may pay for itself in minimizing heat loss.

Tough stuff to figure out in old houses.

That little heater has 100% efficiency(no combustion loss) and 100% distribution efficiency(it's blower into the bathroom space-where it's needed,no loss through supply conduits) Neither efficiency could your central heat(oil, gas) achieve.

In fact if your furnaces efficiencies(both combustion and distribution) are not that high, electric baseboard could be cheaper to run in some spaces in your house. Except you're probably on natural gas which is a huge $ saver over oil. Oil in Maine is nearing the 4.00/ gallon ceiling.