EDIT: Salt has been used for quite a long time, but during the Antiquity, the Middle-Age etc ... it was quite expensive, like a luxury. Nowadays, salt is really common and cheap, when did it became that way ? And how ?
How long have humans been adding salt in meals/to food? written by u/Temponautics
For a little more on salt (and its best friend, pepper) see Why is salt & pepper so universal across American diners? Was this a result of a adversting campaign or a national restaurant culture? When did it began? written by u/SocratesTheBest
On the cheapness of salt At what point in history did salt go from an expensive and rare ingredient to a cheap, commonplace ingredient? written by u/ARandomNameInserted.
This answer doesn't document the change in prices, but discusses the variation prices in regions across regions: I know that in the Middle Ages salt was an extremely valuable product, was it the same in the Mediteranean regions despite being so close to the sea?written by u/wotan_weevil