About $3500 to get the 100-400mm and a 1.4x teleconverter (which are out of stock everywhere) to get the zoom range of the 200-500mm lens. I use the 500mm PF and 600mm f/4 lenses with the FTZ on my Z9 and no issues in doing so. For me it is easier to leave the FTZ on the camera and only change the lenses.
For me the 200-500mm while a great value was not a favorite lens. 200mm was too long and cropped subjects too much too much of the time. I prefer the 100-400mm zoom range and people are getting great results using this lens with the S TC-2x teleconverter that makes the lens a 200-800mm f/11 when desired.