Don't worry we pay about US$7 per gallon over here
I have considered an i3 94ah BEV to drive during the week to complement the M6 but then I remind myself that I bought the M6 because it has 4 seats, is supposed to a do everything car and I can still park it in any parking building.
The money saving isn't quite there for me to bother with an i3 but if fuel gets to US$10 per gallon then I probably will get an i3, 50/50 to keep the mileage low on the M6 and it won't be costing me anything at all to buy the i3 due to the saving I'll be making - I have done 20k miles in a year in the M6!
I personally think a Model S is a good only car, especially in 100kwh battery form. The i3 clearly isn't and is a good second car, but I think it means the other car is an AMG GT or something with 2 seats.