I bought from the dealer and paid $500 for hitch and trim kit.

I wanted it covered by the warantee as the hitch also includes an
anti sway module and connects to the X5 bus.

If you go third party you will probably need to buy a wiring kit in addition
to the hitch.

The BMW wiring kit lists for $235 and includes,
Control module, wiring harness, 7 pin trailer connector, 7 to 4 pin adapter
and hardware kit.