Ask Engadget: best sub-$70 smartphone battery pack?

We know you've got questions, and if you're brave enough to ask the world for answers, here's the outlet to do so. This week's Ask Engadget inquiry is from a nameless Engadgeteer, who needs a heavy-duty smartphone battery for his nomadic life. If you're looking to send in an inquiry of your own, drop us a line at ask [at] engadget [dawt] com.
"I'm kind of a nomad: I travel anywhere I can, sleeping away from civilization in my sleeping bag. My only issue is that my smartphone needs a recharge every two days. What I need, is an external battery pack that can give me a week's worth of juice -- but can I get a lightweight one that costs under $70? Many Thanks!"
This is going to be a tough one, we know -- our first thought was toward a Powerbag with a 6000mAh battery, but that costs $170, more than twice our man's budget. Is there something cheaper you could suggest to our nomadic friend that'll enable him to stay off the grid long enough to become a modern day Grizzly Adams? All we ask for is a comment, and your knowledge to steer it by.