Most of the range reduction is due to the increased internal resistance of the battery at low temperatures. There is also less regeneration at low temps because the battery cannot accept high charging currents when cold without damage.
The cabin heater might cost 10 miles for an hour's use but unless you want sauna conditions is not a big issue. For max range heated seats and a cool cabin are the way forward. Some cars use an air source heat pump for more efficient heating, and also make use of the heat from cooling the motor and electronics. My i3 range will decrease from about 160miles (20 degrees C) to about 120 miles (0 degrees C).
John
