Mwave offers the iKross Rabbit Portable Speaker for Apple iPhone, iPod, or iPad, model no. IKSP12, for $16.99 with free shipping. That's the lowest total price we could find, outside of the mention below, by $18. Features include 3.5mm audio input/output, on/off button, and LED indicator. It's powered by a USB cable or three AAA batteries (batteries not included.)

A close price: BlueMall via Amazon charges $17.99 with free shipping via Prime.