Groupon offers the Ion Party On Wireless Bluetooth Speaker with Built-In Party Lights, model no. ISP43, for $24.99 plus $3.99 for shipping. Outside of the mention below, that's the lowest price we could find by $6, although we saw it for $9 less in March. It's compatible with most Bluetooth-enabled devices and features a built-in rechargeable battery, 3.5mm auxiliary input, and on-board controls.
A close price: TechRabbit via eBay offers it for $29.99 with free shipping.