Adorama offers the Grandmax Mini Tweakers Portable Palm-Size Speakers, model no. SPKR-M2RC-BK, for $12.99 with free shipping. That's the lowest total price we could find by a buck. They feature a 100Hz to 20kHz frequency range, rechargeable battery, and 3.5mm audio plug.
Update: Note that this item is now backordered, but it can still be ordered at this price for delivery when available.