Sure the dealership will cost much more than any aftermarket option. In other luxury cars, this is what I have learned first hand -
Aftermarket or discount key blanks do work fine mechanically. The material is usually substandard compared to OEM. Longevity is usually the major difference. If you have a key fob with keyless ignition, how many times will you use the mechanical key?
I have keyless ignition and I can honestly say that I have never once inserted the key.