what area are you?
we usally charge around £185 + vat to supply and fit standard towbar with single electrics
you can buy the parts yourself from the likes of ebay, if your handy with spanners and ok with electric then its do able yourself, as long as you have the right tools and dont mind pulling your car to bits and have a spare day then you'll have no problems, but they are an involved one to, bearing in mind its, bumper off, lower exhaust, remove rear panels in the boot, boot floor covering and rear sub/amp, the reason you need to remove the interior parts is to get to the wiring and gain access to the nuts that hold part of the bumper on