Double the cost?
Prius starts at $24,200
Volt starts at $26,670
http://www.chevrolet.com/volt-electric-car.html
The Chevy’s price has an asterisk, “price after tax credit”. If the Prius qualifies for the same credit, it isn’t mention on the page you linked to. I may also be thinking of the cost of the Volt I, which I believe cost more than 40K before tax credit. If this new one is less expensive (and a more conventional hybrid), my apologies.