$90 buys a 9V/250mA panel from Dexter Industries, supplying enough juice to power a NXT brick and a single servo.

dSolar Systems allow you to power your Lego Mindstorms NXT robot using nothing but the sun! This 9V solar power system is compatible with Lego Mindstorms NXT creations and includes everything you need to get started. Easily attach the solar panel to any of your Mindstorms NXT creations.

They also sell capacitor banks, battery pack rechargers as well as a “Solar Parallel” that can connect three panels together.