Groupon offers the manufacturer-refurbished Motorola TX500 Bluetooth 2.1 In-Car Speakerphone, model no. 89494N, for $24.99 with free shipping. That's the lowest total price we've seen without the hassle of rebates. (It's a current price low by $5 for this unit in any condition.) It features up to 45 hours of talk time, up to six months of standby time, ambient noise reduction, and connection to two phones simultaneously.
A 90-day Motorola Mobility warranty applies.