Here’s an external battery (available in both white and black) for the iPhone 2G and 3G for only $11.99. The advertised specifications claim 1900 mAh and compatibility with both versions of the iPhone. Although this is one of those “stick it on the end of the phone” batteries that seem inconvenient for long periods of use – you’ll have a hard time finding anything cheaper. Last I looked at Apple their name brand batteries were still in the $99 range.

1900 mAh rechargeable external battery for iPhone 2G/3G