Solatec - A Nevada company that is marketing an add on kit incorporating PV units on the roof of a Prius. The PV units provide battery charging and can boost MPG by up to 10%. The cost of the kit is $2,195 (requires professional installation, which I assume is not included in the price). I would expect to see something like this coming from more companies in the relatively near future.
At the max of 10% improved gas mileage, you would have to drive about 773,000 miles to recoup the cost. This is obviously a solution for those concerned for the environment more than those looking for better gas mileage. Assuming you want to recoup the cost in 100,000 miles, either the price will have to decline or the mpg % improvement increase by a factor of 8 (or some combination thereof). Note, if the batteries could be charged while the car is parked or if the Prius had larger batteries, either of these changes would equate to better gas mileage and would reduce the recoup time.
$2,195 plus $1,000 installation estimate = $3,195 total cost.
$3,195 / $2.50 per gallon of gas estimate = 1,278 gallons of gas equivalent
1,278 gallons of gas * 55 miles per gallon city = 70,290 more miles than at 50 MPG
70,290 miles / 5 mpg improved * 55 mpg actual = 773,190 total miles required to recoup cost
Solar Panel Specifications:
Panel Thickness: 0.6mm
Flexibility: Conforms to the contour of the vehicle.
Adhesive Strength and Durability: Rated for aircraft use
Manufacturer Rated Output Voltage: 18 Volts
Manufacturer Rated Output Current: 30 watts (2 panels)