Keep in mind Gary that if you get near the EPA for the Corolla, you will most likely get near the EPA estimates for the Prius. So you will get close to 50 mpg on the highway (I get 50 mpg on the interstate) and get in the neighborhood of 55-60 in City (I get 60 in the summer). So depending on the amount of driving that you will do in city, the 20-25 mpg difference would make up the $5k much quicker.

However, I would doubt it would truly be worth it to switch from a Corolla to a Prius on gas savings alone. I don't think anyone has potrayed the Prius as the ultimate money saving car. If a person really wants a cheap commuter auto, get a slightly used Echo. The 10-15 mpg increase in mileage with a Prius would take forever to make up the 15K in price.

However, if you needed to step up in size (I can't fit in the Corolla) or want some of amenities not offered on the Corolla, like the nav system, Bluetooth, HID headlights, etc, than moving up may make more sense.