The other logical possibility is that all leaked parts thus far have been for the low cost phone and we haven't seen anything from the 5S yet.
Unlikely I know, but still a possibility.
One benefit of this low cost phone is that it can provide a smokescreen for the flagship phone.
I suppose most internals will be shared between light and 5S, i mean circuits etc
CPU, GPU, embedded memory, screen quality and casing would account for most of cost/price difference