Cafago offers this 10-watt Monocrystalline Silicon Mini Solar Panel Charger for $12.85. Coupon code "ZL1613MG" drops it to $10.89. With free shipping, that's $9 off list and the lowest total price we could find. It has one USB port and is compatible with iOS or Android devices.
Note: This item ships from mainland China and will arrive in two to six weeks.