Stanley SS4LS Simple Start™ 8000 mAh Li-Ion Jump Starter, $15 (In-Store purchase/YMMV); Includes 2A USB wall and car chargers.
Clearance item; Price varies by location. Use links below to locate item. Brickseek reports prices as low as $15 at some locations. Box also contains 2A USB wall & car chargers--over $15 value by itself!