Goods become expensive when there is constrained supply and lots of demand.
Salt has lots of historic uses in food preservation, so I can understand the demand.
The supply however seems almost entirely unconstrained, since anyone with access to saltwater can probably make tens of kilograms of salt in a working day with only firewood (cold climates) or sunlight (equatorial climates), and no special technology.
Restricted access to seawater seems unlikely.
Hence, I can't see any way in which the price of salt could become significant.
What am I missing?
My other response has been linked here, but I'd just like to address one point in particular.
When we talk about salt having a high value, it's important to remember that geographic variations in value were much more pronounced before the introduction of systems for moving large quantities of goods easily. So while a coastal community could easily produce enough salt for itself, a community further inland likely could not, so there would be a steep price difference. This is what motivated trade, and we tend to work out prices from merchants books. The problem is that in the communities that produced salt, we don't have lots of people purchasing from merchants, and vice versa. This tends to skew prices higher.
I'm not a historian here; but I've seen this question answered here recently. So I shall point you to this post by /u/wcspaz.
While, as I understand, it is not against this sub's rule to ask already existing questions, I'd like to remind you that there is a FAQ for this sub. You can also use the search function. It'd be much faster for you to get a answer that way, if a similar question has already been responded to.
Edit: the author of the post is now included in my response.