Soft sleeper curent prices AFAIK are 439RMB for upper bed, 499RMB for lower bed on the Z train and per direction.

Chinese trains have a lot of staff including security people and I have never had anything stolen from me nor have I heard that happening to others on the upscale trains (D and Z).

Just helped someone purchase soft sleeper tickets during the Chinese mid-Autumn holidays when travel peaked and he was still able to snatch those beds.