I have the second generation of the RAVPower external battery (link below) that was recommended to me by an electrical engineer friend. I use it when flying with my iPad Mini and it's now one of the most important things in my flight bag. I picked it up for $29.99 last year when they were having a sale, but would easily pay up to $50 for it again.
http://amzn.com/B00MQSMEEE