1) Check out the real mileage database:

http://www.greenhybrid.com/compare/mileage/

Real owners are getting very similar mileage as a whole group between the Civic and Prius. Currently Civic owners have a median of 46mpg, and Prius is at 48mpg. That's not what I call "much better mileage" really.

2) You're getting a little confusing about the pricing. The Civic is without a doubt a less expensive car. Here in Phoenix one of the local dealers was advertising Civic Hybrids in stock at $18,500. Keep in mind, when you say "base price" it's a little misleading since the only option for the Civic is the CVT. Other than that, it's a fully loaded model. The Prius on the other hand can be optioned up to well into the 20's.

I think that really it comes down to how much you want to spend, and what you're looking to get out of it. The mileage is close enough that if you paid $22.5k for a Prius versus $18.5k for a Civic, you could drive a few million miles and not make up the cost savings from the difference in fuel consumption between the two. If you want a car with a bunch more features and a more high-tech look and feel, then the Prius, although it costs more when optioned up, it would certainly play the role of overtly high tech car better.