$6500.00 to double your hybrid's fuel mileage?

So, you achieve 50 mpg in your Toyota Prius, nice, huh? Would you pay another $6500 to double your mileage? By next summer, that's the price Hymotion is hoping to sell its plug-in kits for hybrid cars. Currently, the price for such a kit and installation is $12,000. Is $6500.00 still too expensive, or a great bargain?