Well, using 28¢/kWh for the M3 and $4.30/gal for the A4 (premium fuel), assuming a primarily highway commute, ≈15k mi/yr, and adding in $250/yr maintenance, I'm counting a difference of $1160/yr. Purchase price of an A4 is $37,400 (base), and a TM3 (base) is $35,000 - $3750 (fed) - $2500 (CA) = $28,750. If you keep it for ten years, the Tesla will save around $20,250 compared to the A4.
If you're set on buying a nice new car, then the TM3 will save you money hand-over-fist compared to a new A4.
On the other hand, if you're trying to pinch as many pennies as possible, then replacing a car you already own usually isn't the right call. However, if your trade-in is worth $10k, then a $13k used EV (LEAF, Soul, e-Golf, i3) will be saving you money after 3 years compared to keeping your old (30mpg, regular fuel) car. Since you're already driving a fairly efficient car, your savings aren't as steep as most people, but the payback period shrinks if you charge at work (commercial rates are usually lower than residential), charge off-peak, or adjust your commute for lower speed.