chenxisen via eBay offers this 4-Port USB Car Charger for $4.99 plus 50 cents for shipping. That's the lowest total price we could find by $4, outside of the price below. This charger is compatible with iPad, iPod, iPhone 3G, 3GS, 4, and more.

A close price, MidnightBox offers a similar charger for $2.95 plus $3.99 for shipping.