[Ecky]

I've calculated back-of-the-envelope that a car A/C unit draws in the range of 600w. A 100w panel runs about $150 on amazon, so a good way to estimate the gains is to find out your mileage loss from running the A/C, divide that by 6 and add it to your normal mileage.

I'd guess it might save you 10-15 gallons of gas per year if you drive a lot, so it'd take about 3 years to pay for itself.