|Well used but still some life left|
Fortunately, the iPhone 3G battery is more or less user replaceable, likely the last model that can say this. $11 for a replacement battery on Amazon that was shipped fast.
The instructions to replace the battery were on iFixIt iPhone 3G Battery Replacement. The instructions were easy, precise, and really don't need any modification, except allow about an hour for the replacement as some of the cable reassembly can take longer than expected.