Anker Direct via eBay offers the Anker PowerCore 10,400mAh Portable USB Charger (pictured), model no. AK-A1214011, for $19.99. Coupon code "XCWT4JFT" cuts it to $15.99. Plus, Prime members bag free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping adds about $7.) That's $26 off list and the lowest total price we could find. It features PowerIQ technology.
Also available is the Anker PowerCore 15,600mAh Portable Charger, model no. AK-A1252011, for $28.99 with the shipping options above. Coupon code "FS9CBYX9" cuts it to $22.99. That's $57 off list and another price low.