You can buy two 10,000maH external chargers of off amazon for $23 each ( EasyAcc EasyAcc 10000mAh Brilliant Ultra Slim Dual USB (2.1A / 1.5A... http://www.amazon.com/dp/B00H9BEC8E/ref=cm_sw_r_udp_awd_TVfYtb10XPEZP). or $48 dollars total. Where as a 15,000maH costs close to $50.
For the $50 one that's 300maH a dollar and for the $23 one that's 434maH a dollar. And if you bought the 10,000maH ones you'd have two just in case you ever broke or lost one. Either way you'd have enough energy to charge your phone multiple times.
(Sorry for formatting it so weirdly, I'm on a mobile)