Tell me about it, I had a 2007 1 series BMW and its transmission computer became dead. Now, I was quoted 4000USD at the dealership (the car was worth about 5000 at that point). I went to a third party specializing in automatic transmission and they almost could do it by swapping the computer from a crashed car.
Turns out it needs an encryption key (who only the dealership has) to get the car to recognize the computer, otherwise it won't even start. I sold it for parts and will never have an automatic BMW again.
According to the transmission guy things have only gotten worse with newer models, particularly Audi/Volkswagen and BMW.