12 euros  

7 x 3 cm high-quality Schott solar module ideal for powering microprocessor applications.

Nominal voltage: 3.6 V, charge current in bright sunlight at nominal voltage: 26 mA, nominal power: 96mW.
Works well for charging 3 rechargeable AA-cells.

You can of course also easily mount your own, but here it is provided in a convenient frame with wires already soldered on (the nickel backside contacts require a special solder).