Hi there, while you wait for new answers you may be interested in these previous, similar threads:

https://www.reddit.com/r/AskHistorians/comments/5opntw/why_was_salt_so_scarce_and_valuable_when_it_is_in/

and

https://www.reddit.com/r/AskHistorians/comments/2zs46f/why_was_salt_such_an_expensive_commodity_in_the/