REI offers the Serfas Trident USB Rear Bike Light for $14.93. Opt for in-store pickup to avoid the $5.99 shipping charge. (Knkdbs via eBay charges the same with free shipping.) That's $15 off list and the lowest price we could find. It features a 35-lumen output, four lighting modes, 3.5-hour charge time, and 1.5-to-8-hour battery life.