Disks are cheap, but disk drives are ~$80. (Consoles are sold at a loss, so this comes directly out of profits.) I don't know how big PS4 games will be, but flash storage could cost 25 cents per GB.
But yea, I think you're right that the optical disks will still end up being cheaper.
A disk is a lot cheaper to stamp and print than flash or some type of ROM would be.