You basically have to look at the availability and cost of transport of the goods. Salt was abundant in the Dead Sea area so the need to import salt would have been pretty low.
As for the movement of writers, or the scrolls post creation, you'd look at where scripts were generated and where they sourced their materials. Just speculating, as I haven't read the background material on this new discovery, but it would make sense that the monasteries in Jerusalem would have a reproducible sourcing process since they would be generating manuscripts often. Same would go for monasteries around the Dead Sea.
As for the movement of writers, or the scrolls post creation, you'd look at where scripts were generated and where they sourced their materials. Just speculating, as I haven't read the background material on this new discovery, but it would make sense that the monasteries in Jerusalem would have a reproducible sourcing process since they would be generating manuscripts often. Same would go for monasteries around the Dead Sea.