Daily Steals offers Mophie Space Pack 16GB Battery Case for Apple iPhone 5/5s/SE in White or Gold for $14.99 with free shipping. That's the lowest price we could find by $3. It features a 1,700mAh battery (enough to double the battery life of your phone) and provides 16GB of additional storage space.
Also available is the 32GB version in Blue for $19.99 with free shipping. That's the lowest price we could find by $10.