You also got to consider the likelihood that as full or majority electric power cars start to become popular, you're going to see governments at the state, local and federal level attempting to make up for lost tax revenues. I'm not sure how they'll manage to differentiate between power for transport and power for home/business use, but if there's one thing that the government is good at it's figuring out new and inventive ways to tax people.
Suffice it to say, driving one of these, especially on a short commute, will really save money, as well as the environment, but don't count on power being (as) cheap for long if we see a lot of these types of cars take to the streets.