External batteries keep getting cheaper. Amazon is selling a Limefuel 20,000mAh battery for $29. The 15,000mAh sells for $25. Both models have 1A and 2.1A output ports.

http://feeds.boingboing.net/~r/boingboing/iBag/~3/gOOVSysHbo8/20000mah-usb-external-battery.html