The fair test is to compare it with buying a conventional vehicle which gets good gas mileage (and costs less to purchase). I decided to check his work (the figures used are explained in McGonigle's essay) by writing a bit in Lisp (Emacs Lisp to be exact).
(let ((gas-price 3.0) ;; dollars/gallon (prius-price 20896.0) ;; US dollars (corolla-price 15551.0) ;; US dollars (prius-mileage 45.0) ;; miles/gallon (corolla-mileage 35.0)) ;; miles/gallon (/ (* prius-mileage corolla-mileage (/ (- prius-price corolla-price) gas-price)) (- prius-mileage corolla-mileage)))
In high school and university science courses , we were told to practice using our units to double check our algebra.
x x dollars - dollars ----------- - ----------- = --------------------- mi/gallon mi/gallon dollars/gallon
Anyway, the conclusion is that buying a Toyota Prius over a Corolla -- a Corolla is 5 thousand dollars less and gets a comparable 35 miles per gallon -- will only break even on gas spending when you've driven your Prius 280612.5 miles. If gas prices sky higher than 3 dollars, then the Prius becomes a surer bet. I've heard of Corollas driving that far, but I don't think there's a Prius old enough to have that kind of mileage.
My sociological conclusion is that the mass demand for hybrid vehicles is by consumers with either strong opinions on the environment or they're misinformed. For either camp, it's another argument for mass transportation. Get on the bus!