Are you talking about how you think it was back then or about how it works in game? In EB only silver and gold mines bring income through mining and they both bring the same amount of money (600/1500 per resource). Salt is only a trade-good and gold only more worth in trade then silver.
IIRC the team said that in many regions in the antiquity silver was more worth then gold (e.g. Egypt) or about the same.