[a humble lich]

For a location where there is regular trade, one way to estimate the cost for ore, gems, etc, is to estimate how much it would cost a merchant to ship goods out, including the profit for the merchant. The cost in the mountain range will be the normal cost minus this. This means for bulky and heavy materials like iron ore you could probably get a pretty good deal, but for light easy to transport materials like gems there is probably not much of a difference, unless there are other factors involved (like if it is dangerous for merchants to travel to the region).

In some locations it might not be possible to buy items direct from the suppliers like this. If you walked up to a diamond mine in modern South Africa and said you wanted to buy some uncut diamonds, they would probably tell you to go to Antwerp like everyone else.