This is pretty consistent with my experience in this, my first winter with the i3. With temps in the 20's and low 30's I lost about 25% of my range. On days where I could pre-condition the battery before leaving the house things were a bit better, but I couldn't always do that due to a non-predictable schedule. And even on days where I could, I would usually drive 10 miles, park the car for an hour or two, and then drive again, so the battery would lose that heat before my second drive. Add to that the need to run the heater for comfort and keeping the windows defrosted and the cold temps definitely take their toll on range. We're now warming up to high 30's and low to mid 40's and I'm seeing an improvement in range already.
Of course the same cold weather driving also causes my ICE vehicles to take a mileage hit, although nowhere near 25%. And when you've got a 400-500 mile range in the tank, you don't notice it and don't care.
2014 BMW i3 with Range Extender Engine, Capparis White/Frozen Black, 20" wheels,, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, TurboCord EVSE