This is 3 of the 4 Xs in the 4X genre.
The main appeal of games like X is exploiting galactic trading routes to the point where you have enough money to build your own mining operations, then your own space stations, then your own trade routes, until eventually you have created your own economy.
Fable has fluctuating item prices based on the item’s relative scarcity in the world. However, much like everything else in Fable, this system was not well thought out, as the value of an item is only determined at the point of sale, not relative to all other items. So all you need to do to generate unlimited money is run around collecting diamonds until you have about 100, sell them all to a merchant with no diamonds in a single transaction, and then immediately buy them back in a single transaction. When selling them their value will be appraised at maximum scarcity level, and when rebuying them they will be appraised at their minimum scarcity value.