I paid $15.34 for 59 kWh ($0.26/kWh according to the receipt), which at the conversion rate you describe of 3mi/kWh comes out to 177 miles.

Using your example of 32mpg and $3.64/gallon, 177 miles would cost $20.13.

So roughly 25% cheaper for the supercharger vs gas. Bear in mind filling up gas takes five minutes whereas the supercharger took over two hours.

Good point about charging at home overnight though.