The Good Blokes via eBay offers the refurbished Mophie Juice Pack Case for HTC One M7 in Black, model no. JP-HTC-ONE-BLK, for $11.99 with free shipping. That's the lowest total price we could find for a refurb by $13. It features a 2,500mAh battery and purports to add up to 8 additional hours of talk time.
A 90-day Mophie warranty applies.