Does it really matter what the building description says? You can see the exact amount in the city details screen. I think it depends on the number of actual mines in the region. For example, i think Pella has 3 gold mines. So it makes a large sum of money. Other regions have only 1 or 2 mines and they make less. Of course silver is less again and salt I guess would be less. Again, does it matter what the building description says? Really?