I actually read a lot about ancient economies, especially during the medieval era. Yeah, I’m a geek, but you’re a role player, so can you throw stones? Anyway, one of my favorite books on the subject mentioned something that has had an impact on me for some time now: It was important where they mined the silver they used to make coins.
OK, this seems logical and all, but it is actually a lot more important than you might think. Just as happens today, the “world’s currency” (today considered to be the US Dollar) gets to have a far bigger impact on the value of things than the other guy’s get to. What does this mean in your game world? Well, if most of the world’s silver is coming from one region and most folks use silver to make coins, then what the silver miners think something is worth is most likely what it’s worth, even if the iron miners think differently. Why? Well, because people have to trade with the silver miners in order to get their silver. Oh, you can trade your stuff to the iron miners to get silver from them, but that’s adding middle men and is never going to be your best deal. The same holds true if gold is your main currency - the gold miners hold huge influence.
So what? Assuming you don’t just trust your FRPG rule book to tell you what things are worth in your world (and OMG please don’t), then it matters that the silver miners prefer caraway and cinnamon to mace and cayenne pepper. It matters if they think that rubies are better than sapphires, even though chemically they’re practically the same thing. It matters far less if there are silver mines in every country of your world, because then their influence is distributed.
I have another point to make, but I want to avoid getting too deep into monetary policy: The amount of “money” in the world can be measured in many different ways. Modern countries do not agree on what constitutes money, even when each of them has 5+ measurements. Because of that, it is really tough to say this, but ... Only about 10% of the “money” in the US is actually currency as you would see it. How? OK, let’s take a fantasy example: Ben the merchant gives Alan the store owner sixteen bolts of wool cloth. In exchange, Alan gives him a “check” that Ben can cash back in the capital when he sees Alan’s cousin Dean. (Dean will give Ben coins for the check.) While Ben is traveling to the capital, he thinks he has 100 gold coins, because he has the check. Meanwhile, Dean thinks he has 100 gold coins, because he hasn’t made good on the check. Did Alan create 100 gold coins? Of course not, but now the world thinks it is 100 gold coins wealthier than it was before he wrote that check.
Again, so what? While I cannot believe that a fantasy era society would have 90% of its “money” wrapped up in deposits and other “digital” era creations, there still are questions that you might want to answer. Are there banks that lend money to people so they can buy homes, and did those banks get that money by taking deposits from other people? How much does it cost to borrow and how much does it pay to invest/deposit? Or, do the nobles hold all power of loaning money like this because they are the only ones with the money to lend? In Rhum (Fletnern), it is illegal to use the money given to you for safekeeping (on deposit), so they will never have a bank failure, but very few people can afford to buy homes (everybody rents). You may not care about a lot of this stuff, but it does have an impact. Let me give you some mission ideas:
A bank holds the deposits of all the members of the _____ guild and invests them by lending to merchants who trade via ocean going ships. What happens when a ship is taken by pirates? A huge amount of the guild’s money (people’s nest eggs) has just been stolen and the bank is not going to get it back without help from some adventurers. Seems like a “normal” retrieve loot adventure, right? Well, the bank doesn’t want anyone to know what happened, so it has to be kept quiet. Plus, the bank is going to lie to the adventurers and overstate the cargo of the ship hoping that they manage to bring back the ship and more of the pirate’s loot than the ship originally had. If the party does, who gets that profit? Should the party? the bank? the depositors? the original owners? It can get complicated! If the ship sank, are there sea laws as to who owns the cargo while it is on the bottom of the ocean? Today these laws can get pretty sticky, and the local “king” may think the cargo is his. (or the merman king might think it’s his.)
What if Ben gives Alan’s check to the party as payment for something? They go to the capital only to find out that Dean doesn’t have a brother named Alan and refuses to honor the check. Maybe they’re brothers and maybe they’re not. Maybe Ben is the thief or maybe Alan is, or perhaps Dean. The party wants their money but may have to do some serious investigating to figure out who it is that is cheating them. Then they can kill everyone in the house, because we know that’s what adventurers do, right?
Keep thinking about how this can affect your game world. We’re talking about a lot of money here, and a lot of money means a lot of opportunity for paying adventurers.