Amazon with LaptopMate has the Anker Astro3 Dual USB Output 10000mAh External Battery Pack for $50 with free shipping. Features 5V/2A output for tablets and 5V/1A output for phones, two USB ports and has enough power to recharge most smart phones over 6 times.
I recently purchased the Astro 3e and it works great and really comes in handy.