I am going to start by making an assumption: your fantasy game takes place in a pseudo-feudalistic society. Fun fact: so does mine. I have found, however, that far too often the economics presented in fantasy roleplaying games leave much to be desired. For one, the costs of certain goods and services make little sense in relation to others (why does it cost one gold to stay at an inn? Ludicrous.). If you want your game to have a somewhat realistic economy, then you will need to divorce yourself from you modern notions of economics, since these make little sense given the assumed social order of your game’s society.
Let me digress a moment and talk explicitly about some basics of the feudal society. The first assumption is that the majority of common-folk will be serfs of some description. They will not own land, they will have to tithe portions of their harvests to the local lord and various religious institutions, they will not possess much 9if any) currency, and their general day to day activities will be based around agriculture. Inside towns, you will find craftsmen of various sorts, and it will be up to the DM to decide whether or not these individuals are also serfs (not uncommon at all in the manor system). Mass production and market validation of labor will not be factors in a pre-capitalist economy. Cottage industry will be the norm, and will operate largely on a command basis. Outside of the most sprawling metropolises (which probably have a more mercantilist economy), local artisans will not necessarily have a large stock of products to purchase.
Most such products will be made upon request and could take several days; which is useful if the party needs to pursue downtime activities. Artisans and their families will live either above or behind their shops (thus the term “cottage industry”), and those looking to procure goods and services will likely encounter a young son who is acting as an ersatz apprentice. Almost all products available in town will have been produced locally. Occasionally a travelling merchant may come through town to offer goods not normally available, and, more importantly, buy the primary goods produced by the locals (ore, lumber, grain, etc.), but these instances are likely to take place once a month or so in any given town. Majority of commerce will conducted through barter since few will have actual currency. Life in a rural, feudal town would be quite austere.
But PJ, you say, we are in a sprawling metropolis. That’s all well and good, but the market in those towns would still be different from what you are used to growing up in the 20th and 21st centuries. Trade would be highly controlled. Large guilds would set the prices of various goods and determine the qualifications and course of study for those who wish to practice a craft or trade. Within cities, it is likely that the hereditary transmission of trades will be slightly less prevalent because of the guild system. Guilds will be politically powerful and extremely wealthy. While the governor of the city may be appointed by the crown, you can bet there will be a city council made up of appointed (or perhaps elected) officials of the burgher class. Since there is very little production of primary goods inside cities, most people will possess some amount of currency and barter will very rarely be used as a result.
Market competition was not a factor of this kind of mercantilist system (international competition, however, was), so while two different cobblers may have aesthetically different looking shoes, the prices of those shoes will be the same based on the rates set by the guild; woe to the craftsman who produces goods without guild sanction or violates the price controls. Merchants within the city will have goods from near and far in some abundance, but due to tariffs and non-tariff barriers to trade goods imported from other nations will be very costly.
Now that I’ve addressed economics on the macro-level in your game, let’s turn to what you, the DM, can do to limit the amount of currency your PCs have. It would not due to have PCs acquire too much wealth, since a sudden influx of gold could greatly unbalance local economies (see: Spain post-Siglo de Oro)
This is the easiest way to keep your PCs from acquiring too much loot. I am, in general, a proponent and enforcer of encumbrance rules (yes, the fighter with 18/00 strength should be able to carry more than the wizard with a strength of 9!). Since PCs inevitably carry lots of different items, their capacity to carry bags upon bags of gold coins would be quite limited. Because PCs are ostensibly murder hobos, they will not have a place to store their ill-gotten (adventuring is theft, after all) gains. Banking will be unavailable in all but the largest localities, and even then it will be primitive and inconvenient; PCs will either need to go back to the same bank or have a letter of credit drafted by the bank in order to get money from other banks. Treasure left unattended in a keep or dungeon that has had all the denizens slaughtered will be ripe for the picking, and PCs can pretty much count on all uncollected treasure being taken by bandits, other monstrous humanoids, or locals who became curious when PCs started throwing gold pieces around like it was going out of style.
2. Be stingy
Another way to keep the player characters from accumulating too much money is to simply dole less of it out to them. Random encounters do not need to drop treasure, for instance. When you run into mooks in a dungeon what is the likelihood they are carrying any treasure on them? Chances are that they are not carrying much of anything. Admittedly, players hate it when DMs do this, but the upside is that if your PCs get used to receiving little treasure from encounters, they will learn to rejoice over whatever they find.
All towns have some form of government, and most assuredly that government taxes its population. With that being said it is likely that local governments would want to tax rich adventurers when they come into town. Paying taxes on rooms in the inn, on selling their goods at local stores, on docking ships, on salvage from the area, and on property bought. The key to remember is that everyone is going to take a cut on the way up the chain, so if the local lord establishes a 20% tax on salvaged goods you can bet that the mayor will tack on an extra 5-10%, the clerk an extra 3-7%, and the tax collector and extra 2-5% (the DM can feel free to assign different percentages, these are merely illustrative examples).
4. Silver Standard
This is an idea I got from the article “A player character and his money…” by Lewis Pulsipher from Dragon #74. Since I cannot do a better job of explaining this concept than Mr. Pulsipher, I will quote the article directly:
In any description of a hoard of monetary treasure, replace the word “gold” with “silver.” (But don’t change prices or values given for goods or services.) Adopt the “silver standard” which actually prevailed in late medieval times. A gold piece (arbitrarily set equal to 10 silver pieces to make calculations easy) becomes really valuable. And silver, once sneered at as “too cheap to carry,” takes its rightful place as the wealthy man’s mode of exchange. (p. 50)
As you can see, this will help to radically decrease the amount of treasure that players are able to accumulate. Also, this will help make finding mundane armor and weaponry very exciting to the players since such things are very valuable under the silver standard.
Pulsipher, Lewis. "A Player Character and His Money..." Dragon June 1983: 50-54. Print.