There are 2 OEM tow bar options, removable and electric fold away.

http://bmwfans.info/parts-catalog/F3...le/#6879427_12


OEM Prices (CarParts-Pros)
(The kit might come cheaper than individual part prices)

Bar to attach to bumper carriers: €550
Removable Ball: €460
Control Unit: €122
Socket: €77
Socket Holder: €25
Adapter for Socket: €25

There's a load of other smaller stuff needed, but I reckon you'd need the module at least even with a non OEM set up.