Woot offers Apexel 20-40x Telephoto Zoom Lens for Smartphones for $42.99 (orig. $69.99).  Free shipping for Prime Members.

Product Description:
Zoom into far away objects while retaining image quality with this 20-40x Telephoto Zoom Lens for Smartphones from Apexel. Constructed from multi-coated Lanthanide optical glass with BK4 prism, this lens mimics that of a DSLR lens, allowing you to zoom in an out on your subject. The optical glass is ideal for smartphones that tend to offer minimal digital zoom abilities. The lens attaches to most any smartphone via included detachable clamps that screw into place. At its maximum 40x zoom, you can shoot scenes from up to half a mile away.
Key Features:

•   Shoot Scenes Up to Half a Mile Away
•   Detachable Clamps Work on Most Phones
•   20 to 40x Telephoto Zoom
•   Multi-Coated Lanthanide Optical Glass
•   BK4 Prism