On normal wheels, £35-£50 per wheel. If you have the Z4 18" split rim wheel, then quite a bit more, and take your time find someone who knows how to do them properly.
As above mate and also make sure you get them done as a set as you `may` get a colour differance. Also price all depends on the finish of your wheels. Some are painted, others are dimond cut. I'm not sure which but one is more expensive than the other.