REI Co-Op offers the NiteRider Solas 100 Rear Bike Light for $27.93. (Amazon charges pennies more.) Opt for in-store pickup to avoid the $5.99 shipping charge. That's the lowest in-stock price we could find by $2. It's USB-rechargeable and features four modes, including Daylight Visible Flash (DVF) and group ride mode.

Update: Amazon now offers for it $27 with free shipping.