If your primary concern is cost-per-mile, it's hard to beat an old $250 moped off Craigslist that gets 100+ mpg.

If you claim you need something more enclosed, an old truck for $2000 might be your answer. In that rustbucket, you're belching smoke into the atmosphere and burning a dollar of gas every 5 miles, but it will still get you more than 100,000 miles before you match the purchase price of the Prius. In those same 100,000 miles, the Prius has been burning gas as well (true, at a whopping 47 mpg) but this gas cost would be enough to buy you a replacement old truck (or, let's face it, a second old truck).

To be more serious, I bought my 2003 Toyota Matrix XRS with 150k miles for $3100, and have been getting about 30 mpg for the past 60,000 miles. Every purchase of a new vehicle - hybrid, electric, or gas - is done for non-economic reasons.