Many people assume that they can buy a Carolla or a Civic and their environmental impact is lower than a Prius - this is not true.
With your reasoning, then why would anyone pay more for any car than they would for a Toyota Corolla? A Prius and a Corolla are not similar cars at all. Lots of people pay tens of thousands of dollars for a BMW, Lexus or (gulp) Cadillac badge and get no real value from it. At least a Prius is a very comfortable mid-sized car (Corolla is a lot smaller!), has amazing computer display that shows you how the car is working and plenty of storage space in the hatch-back.
What I would like to see is a Corolla hybrid - then you could make a similar comparison. Honda is still working on the Fit Hybrid which is also interesting as it's expected to retail in the same range as a Corolla and fits a similar compact car profile but Honda's hybrid technology is not as good as Toyota's. More hybrid or 100% electric choices would help.