Emergency power charger iPhone 3G

I have an iPhone 3G. Im doing a camping trip this summer for about 10 days and Im trying to find a cell phone charger that can charge without a car/wall outlet/computer etc. I read some reviews on CNET for crank chargers and they didnt seem reasonable. Im not interested in spending money on a solar power charger (they dont seem that great either...) unless I absolutely have to.
Anybody out there tried any they found that worked?
I did see some like this:
http://www.google.com/products?client=safari&rls=en-us&q=emergency+iphone+3G+charger&oe=UTF-8&um=1&ie=UTF-8&ei=O773Sd7hLKDGMoHY7csP&sa=X&oi=product_result_group&resnum=1&ct=title
Not sure they really work since some of them only cost $4.00. Couldnt find anything iPhone 3G specific on Amazon.com...
Thoughts/Suggestions/Advice?
Thanks for your help!
IPHONE 3G