uNu Power 1700mAh Battery Case for iPhone 4 / 4S for $20 + $5 s&h
Moofi (Woot's secret site) offers the uNu Power DX Plus 1700mAh Rechargeable Battery Case for iPhone 4 and 4S in Matte Black or Glossy White for $19.99 plus $5 for shipping. That's the lowest total price we could find for this case in any color by $6. It purports to double an iPhone's battery life. Deal ends October 19.
A very similar deal was also reported by: