The Tesla is never going to be a winner on cost alone and no Tesla buyer is getting it under any illusion of saving money. It's "rational" to buy the car, relative to a normal Elise, if you feel that the electric drivetrain (and the general exclusivity of thing) is worth the premium. For the dot-com millionaires who appear to be the target market, the marginal cost per mile arguments are there as much for cocktail chatter as to make a business case for future electric cars, when they can get down the production costs.

Personally, I'm waiting for Jeremy Clarkson to get his hands on one and tear it to bits on Top Gear. You know they would do some sort of side-to-side comparison with a standard Elise (or maybe one with an aftermarket turbo or something, so the 0-60 times are closer).