I've seen in numerous sources that salt used to be very expensive, even worth more than its weight in gold, and a big part of trade all over the world for much of history. But then I look at Diocletian's Edict of Maximum Prices, a very useful resource for prices in the late Roman empire. It tells us that a modius (8 liters) of salt was worth 100 DC. An ordinary worker would make 25 DC per day, so that's 2 liters of salt, which is about 4 kg.
Granted, that's a lot more expensive than today, when a worker's daily wage could easily buy you 100 kg of salt (probably 1000 if you buy in bulk). But still, 4 kg per day, that's hardly "worth more than its weight in gold", right? In fact, in the list of grains mentioned in the edict, a lot of them have about the same price - wheat, for example, is also 100 DC per modius. And since the density of wheat is several times lower, salt wasn't even worth its weight in wheat.
So how does this add up? Why did people even bother trading in salt if it was that cheap? Or was is just unusually cheap in 301?
Salt was an important trade item in several periods of history--another example is the salt boom in Sichuan during the Song Dynasty, which led to the large scale exploitation of natural gas. However, its importance was not as a luxury item, as you note it really was not that expensive. Rather, it was an item of fairly uniform quality, fairly easily gathered, and with a vast potential market--in other words, it was a perfect bulk trade item, like wheat, wine and olive oil. Bulk trade items are less visible to researchers than high value luxury goods, but increasingly researchers are coming to understand its importance to even the seemingly most primitive trade networks.