Salt has a base yield of +1 and +1, which is by default far superior to +2 of normal luxuries. Salt is improved by mine, which is easily accesible, and the mine improvement gives you +1 AND +1 (!?), which is again much superior to the normal +1 most luxuries get when improved with plantation. Chemistry will increase mine value with another +1 production. This basically means that from ancient era, a Salt/plains tile will have a yield of 3 2 1. And then additionally you get the benefits of any normal luxury, ie. nationwide happiness and furthermore can trade it for gold or other luxes. Salt/desert tiles obviously are less attractive, but still, obviously better than Ivory/desert or Incense/desert tiles. It's a mystery to me how game doesn't disciminate between salt and the other luxury resources, just like it does with Marble. Either they should have made it like that, or just a regular luxury (i.e. gold yields).