Here is a cool project that you could build for about $20.00. Check out the full build details.

“This project details a small & simple, but very powerful USB charger for your mp3 player, camera, cell phone, and any other gadget you can plug into a USB port to charge!

The charger circuitry and 2 AA batteries fit into an Altoids gum tin, and will run your iPod for hours! 2.5x more than you’d get from a 9V USB charger! (See Process for math/calculations) You can use rechargable batteries too.”