You do not give any info as to which year/model you are talking about. For later E70 models(LCI) I would suggest biting the bullet and going to the dealer as I found out the hard way this is the cheapest alternative.
Cloning a key can brick you CAS module meaning you need to have the vehicle flat bedded to the dealer and a new CAS module installed and programmed. That was around $1300 in my case. Then I still required a key that cost $540 from the dealer.
Your money, your choice.

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