I switched an older 318ti from a manual to an auto, and it wasn't cheap. And there is far less required for the old auto's compared to the M-DCT. Time alone in a shop could be 2-3 thousand not to mention the price of the tranny, which i bet is super expensive. You would be way better just trading the car in or selling it.
I will pay more than 25,000$, so if i will pay 5000$ for the gear it is better than buying a new M3.