BlueRigger via Amazon offers the Kinivo Bluetooth Hands-Free Car Kit, model no. BTC450, for $39.99. Coupon code "35BTC450" cuts it to $35. With free shipping, that's the lowest total price we could find by $3, although we saw it for $10 less in July. It's compatible with Bluetooth cell phones and Apple iPhones and features a microphone, music controls, and adhesive pad for mounting.
Note that this car kit requires a 3.5mm input jack in the car stereo.