the way the cells are arranged here you're getting 20400mAh @3.7vdc; or 75.48 watts. At 4.2vdc USB charging voltage that's just under 18000mAh for a 6-cell device. $45+$17 = $62 for this setup.
Or I could buy three of these:
http://www.amazon.com/Shockproof-Flashlight-Rechargeable-Battery-Waterproof-Smartphones/dp/B00MG9VVW4/ref=sr_1_1?ie=UTF8&qid=1425217678&sr=8-1&keywords=waterproof+backup+battery and get a lot more mAh.