Do the math on the price of a Toyota Corolla vs Prius. Then calculate the price diffence against the small amount of milage you gain with the Prius.
It would take you 20 years to recoup the difference.
Whatever rationale they might try to sell, people drive a Prius for the same, simple reason a lot of people drive whatever they drive; they're trying to impress someone. "Look at me, I'm green!"
They're not much different than those whose cars say, "Look at me, covet this!" or, "Look at me, mine's bigger'n yours!" Just a different target audience.
Not that there's anything wrong with that. Freedom of choice is a good thing.