Note: if you're reading this on Facebook, keep in mind that it's imported from my blog, "Things Which Bore People", which is exactly what it sounds like.
There really isn't such a thing as economics in D&D. All items are given a static price, regardless of scarcity. For example, a player couldn't buy up all of the suits of magical armor in a large city then sell them at a profit because they have a monopoly. This is basic gameplay/story segregation, because having to keep track of supply and demand in a fantasy world doesn't really add anything to the game. That said, it's something that still bothers me, because there are some underlying assumptions about the world that it should logically affect.
For example, why are there farmers? The rulebooks have two different spells to make food; "Traveler's Feast", which provides food for five people for 24 hours with a cost of 35gp, and "Bloom" which causes crops to bear fruit, which feeds five people for a week with a cost of 20gp. In the medieval era, even going into the early modern era, the vast majority of the population worked at farming. This can't be the case in D&D, unless labor is worth a lot less; the only reason there would be farmers is if buying the food would cost less than conjuring some up, which would only be the case if farm labor were worth practically nothing. "Bloom" takes 10 minutes to cast, and feeds five people for a week. The labor of five farmers who are working for a week must then be worth less than the 20gp it costs for the spell, which means that they must individually be earning around 1gp for every two days of work (5sp). This doesn't make a whole lot of sense in comparison to how much things cost, unless the players are getting insanely gouged by everyone they meet (which, rules-as-written, they are, but not to that extent).
There are a couple of ways to reconcile this; the first is to make magic really rare, so that it doesn't affect much of the mundane happenings. This doesn't do much for a variety of other problems - bringing back a dragon's hoard should still make prices shoot through the roof as gold is devalued.
That brings us to the second solution; create an elaborate spreadsheet attached to a random number generator which will alter prices based on a variety of factors. This is better, but the amount of work it would require compared to how much it would improve the game means that it's not really worth it.
A third solution is to alter your game world to make most of the logical problems go away; it wouldn't make sense for there to be farmers, so there aren't. Unfortunately, this has a tendency to reduce a setting to magitek, because these people would be using magic like we use technology. The industrialized world has less than a half of a percent of its population working farms, because the technology saves on labor. The rest of the economy would focus on industry, entertainment, and war (as it does now) - and healthcare would be even better than in the real world.
But what I've decided to do this time around is to compensate for the players. Basically, the players bring in a lot of money, which causes inflation. But at the same time, magic is advancing, which increases productivity, which causes deflation. It will work out to be the case that inflation and deflation keep exact pace with each other, which means that the prices listed in the books don't have to be altered. And in this way, the players will see the world change around them; when they're level 1, a +1 magic sword will be a fairly rare thing, but when they're level 10 a +1 magic sword will be a weapon weilded by commoners. So basically, the players will be living through their own version of the Industrial Revolution, and while their growth in power will outpace the world's, it won't be a case of quadratic expansion against a static realm. Hopefully this will help my suspension of disbelief.