Yugster offers the Urge Basics Armorlite 2,400mAh Battery Case for iPhone 6/6s in five colors for $12.97. Coupon code "FreeYugBuck" cuts it to $11.97. With $4 for shipping, that's the lowest total price we could find by $9. It features up to eight hours of talk time, charges within two hours, short-circuit and over-charge protection, and 2,400mAh lithium ion battery.
Note: The countdown timer on the product page does not reflect an actual expiration time and resets daily.