Adorama offers the Energizer Energi-To-Go Audio Power Pack for iPod, model no. IPODPOW, for $3.99 with free shipping. That's the lowest total price we could find by a buck. It works with any dockable iPod and includes a reusable charger, various connector tips for different iPods, and requires four AA batteries (included). It's also compatible with select cell phones and gaming devices.
Audiovox Energi to Go Charger, a similar product, is available at these stores