Yes, that's roughly the way it breaks down for road tripping. Tesla seems to be responding to lower gas prices by reducing their supercharger prices to keep things about even. I know I paid more / Kwh on the trip down last fall. As you say, the real savings in fuel cost is for home charging, where the juice goes for less when it comes from the mains, or even free when you have a solar array. Convenience is another big win because you spend about zero time "fueling", as opposed to making a trip to the gas station if you are driving less than the battery range in a day.