That's a simple and complex question all at the same time. Electric rates vary across the country, and if you're like some of us, you're actually getting your electrons directly from sunlight via your roof-top solar array.
Your friends who are still - sigh - driving on fossil fuels can figure their cost much more easily, since the per gallon price is posted at every gas station. Fortunately, the U.S. Dept. of Energy has created a nifty little algorithm so that you can plug in your state's average electricity cost (available on a drop down) and get a good idea of how much it's costing you to drive cleanly, and electrically around town vs. a gallon of unleaded.
One of the other benefits to driving electric is that electric costs don't vary as wildly as fossil fuel costs do - with big spikes and dips according to the geopolitical landscape and oil prices.
Head on over to the Energy.gov e-Gallon website today, and see how much you're saving.