Salt is a little too strong early in the default game.  When improved, it produces more yields than any other luxury except gems.  Not only that, but a majority of those yields arent just gold, theyre the more powerful ones, food and production.  When its present on a plains tile, it has very high yields.

There are two ways bonus yields are applied to a resource upon improvement.

1) The resource receives its bonus from the improvement type, which isnt resource-specific.  This is the case with things like mines and farms, since these improvements not only improve resources, they also improve regular tiles as well and need to have a bonus built into them.

2) The resource receives its bonus from improving itself, which is a unique combination of the improvement type and resource type.  This is the case with things like plantations and fishing boats, since these improvements only improve resources and arent allowed to be placed on regular tiles.

Luxuries normally pick one method or the other.  This isnt the case with salt, and it may have been overlooked during development.  Salt receives the +1P from the mine used to improve it AND +1F for improving it.  
 
Ive included some comparisons in the chart above.  Copper shows how a mine luxury normally receives its bonus yields from a regular improvement.  Crab and citrus show how a luxury normally receives its bonus yields from a special improvement.

Ive modded salt so that the +1F from improving the resource is removed and only the bonus from  the mine is applied.
