You should be able to find the X5 e53 OEM hitch for $495.. Includes the wiring harness and everything you need. You can also install yourself in a couple of hours if you have basic mechanical skills.. There is a great DIY write up on this forum.. I did mine and was not very difficult..

I do not believe you will need BMW to code anything.. I believe the X will recognize the controll box once you plug it in to the harness next to the battery. Mine worked perfect as soon as I was finished installing it... screw the dealer and do it yourself!