Costco has 2 packs of ubiolabs 10,000mAh power banks for $29, making them $15 each, I just bought 2 - 2 packs to hand out for Christmas gifts... no thanks.
Theocratdo make sure to check the voltages that they are rated at. some companies rate at 3.7V, rather than the operating voltage of 5V to boost the mah rating