How do you add Bluetooth to a C240/ 2004 no phone installed. Don't want to spend too much if possible!

How do you add Bluetooth to a C240/ 2004 no phone installed. Don't want to spend too much if possible!

The car I just purchased  (2004 C240) has a telephone feature on the steering wheel, but I am told it would cost $400 to install a factory Bluetooth component to allow the use of this feature.  Are they crazy???  

 

Someone please help!!!!!!!!!