I hacked together a usb solar charger for about $70 using this instructables as a guide (link). I did swap out the LiPo charger they recommended with a new product (link) from SparkFun that's supposed to optimize your solar output in non-optimal conditions. If you can solder (anyone can) you can do this on the cheap. Parts I used:

0.45W Solar Cell (link)
Solar Charger (link)
2000 mAh LiPo battery (link)
MintyBoost Kit (link)

The Instructables says 20 hours to charge the battery with this setup. You can spend a little more money to get a larger solar panel that will significantly reduce charge times - anything that sparkfun sells will work. The Battery is roughly the same size as your typical smartphone battery.