The German-made soft luggage for this bike is generally of high quality, but exorbitant price. I recommend you see the Luggage section of this web site for some American alternatives.
See the general section on Luggage for links to Jesse and Happy Trails. They make aluminum saddlebags for this bike.
<4>BMW GS Saddlebags, Part#71607670827828. Adjust for wide and narrow. Left side 28.5 or 37.5 liters, right side 37 or 46 liters. Aluminum side panels add stiffness. About $750