We have reasonable data for Roman salt prices. Generally, salt was about the same price as wheat, by volume (and therefore, cheaper by weight, as pointed out by u/Zecnic in the https://www.reddit.com/r/AskHistorians/comments/3pnjcq/why_was_salt_so_important_in_trade_was_it_ever/ (and the point of the discussion is the importance of the salt trade despite it not being a very expensive commodity)). If poor Romans couldn't afford salt, they couldn't afford to buy food, either.

Of course, farmers can grow their own food, but might not have access to salt. This is one reason why salt was traded (another is the large-scale use of salt for food preservation). The current US Dietary Guidelines for Americans recommends a maximum of about 1 teaspoon of salt per day, and minimum requirement is approximately 1/5 of that. From the earlier question linked above, a Roman worker could buy 2 litres of salt with one day's salary, which would last over a year if consumed at the Dietary Guidelines for Americans rate, or over 5 years if consumed at the minimum rate. Further, not all of the minimum sodium requirement in the diet needs to come from salt added to the food - some is already present in foods.

Sufficient calories and sufficient protein were a much bigger problem for the poor than sufficient salt.