Id be curious to see what the MPG difference would be if the Prius was offered as a non-hybrid. The battery isnt the only thing unique about that vehicle.
The extremely low coefficient of drag (albeit an ugly design, imo) plus small engine and efficient transmission accounts for the majority of the fuel savings. If you arent in constant stop and go traffic the increase in value isnt as huge.
Ill be saving up for my small car Ill be buying in a few years. A used Toyota yaris, hopefully! That way I can keep my beautiful, but sadly inefficient 20mpg PT Cruiser for the weekends
(not to mention save on the wear and tear).