The SB-400 has already been listed as discontinued at several camera sotes. Nikon SB-500 will replace the already-discontinued SB-400 AF Speedlight Flash.
If you make a search for “SB-500” on Nikon site, you will see that at some point the flash unit has been mentioned at some point on Nikon USA website. SB-500 speedlight price range expected to be the same as the current SB-400.
Nikon SB-500 Speedlight Flash To Be Announced with D750
The image above is the search result from the Nikon Official Website. In addition to the D750 DSLR camera, Nikon expected to announce a new SB-500 Speedlight flash at Photokina show.