A brand new Prius C starts off at around $19,600 according to Toyota website, so I would imagine a used one would be $2,000 - $3,000 less than that. It is a hatchback, and gets 45+ MPG.
I would definitely pick a used, good condition, low mileage Prius C over a new Mirage. If I was on the road a lot (20k miles a year is a lot to me) I wouldn't feel comfortable with the tiny Mitsubishi dealership network. If he broke down somewhere, not even a remote area, it could be a really, really far tow to a Mitsubishi dealer.
Just food for thought.