The cost to add Bluetooth to your car is fairly affordable, typically ranging from $150 to $300. Parts alone may be upwards of $200 and installation can range from free (doubtful but not unheard off), to standard labor rates. Labor time should be in the range of ½ hour to an hour and is usually done at the location you purchase your Bluetooth at. Some dealers offer discounts on installation to get you to buy from them, so shop around.
For more info, see How to Add Bluetooth to Your Car Stereo