Tanga offers the Goal Zero Rock Out 2 Solar Power Speaker in four colors, model no. 94007, for $39.99. Coupon code "CHEAPSKATE" cuts it to $34.99. With $1.99 for shipping, that's the lowest total price we could find by $6. It features a water-resistant shell and 20 hours of play time per charge. It charges from the sun or via USB.
Update: Specs corrected.