Yugster offers the Monster PowerCard 1,650mAh External Battery Pack in five colors for $8.97. Coupon code "FreeYugBuck" cuts it to $7.97. With $2 for shipping, that's the lowest total price we could find by $10. It fits in your wallet and features a USB cable and LED indicator. It purports to provide up to five hours of energy to your smartphone.
Note: The countdown timer on the product page does not reflect an actual expiration time and resets daily.