I think the real point is these kinds of rules are REALLY easy to create. They don't tend to require any kind of elaborate mechanics or even really balance much with anything else. I think the very first JG supplement for D&D was exactly that, and while many people have written the same thing since it always really just varies a bit in terms of how it hooks to the core mechanics of whatever game it happens to be for. In other words, it is fairly low priority because it is both dirt simple to whip up and kind of dependent on the type of game you want. I'd think it would be best as a part of say a PoL supplement.
The thing about 'D&D economics' isn't even that there really isn't any (beyond some very basic illusion) but that there's no one-size-fits-all version you can do except something in most respects just about like the 4e version, totally gamist. I mean you can buy that 3PP supplement that is like 80 pages on medieval economics, but even THAT is a crude oversimplification. Economics done to any degree of complexity means you have to map out the entire system of social roles and obligations, demographics, production, etc. You need a highly detailed realistic sort of world for that.
I'm not so sure that they are easy to create well, but you're right about there not being a "one-size-fits-all" version.
The point of an economic sub-system is not to simulate the economics of the fantasy world, but to create a game-viable and believable system for the PCs interacting with the economics of the fantasy world. In an adventure-only campaign, 95% of the PC-economy interaction is about buying and selling magic items. Correctly, that's the focus of the basic game, and I think the basic 4e rules do an OK job. That is, it's easy to figure out how much a magic item costs and due to the realistic-yet-not-believable-to-many "sell at 20%" rule, the game leaves magic item creation in the hands of the GM while allowing the PCs to craft.
(That said, I think I'll prefer the common-uncommon-rare rule which would allow the PCs to create or purchase certain types of magic items while leaving other types in the hands of the GM.)
The more interesting, if harder to write, form of economics rules would provide an entirely different way of interacting with the game world. Under these rules, PCs can purchase game-world elements (such as inns, castles, ships, etc...) that would provide an in-game benefit. The trick to this system is that it is harder to generalize because (A) the types of things PCs can buy, (B) the degree of benefit and (C) the level of detail depend greatly on the campaign.
This is trickier because the whole idea is that the PCs should be able to invest for future economic gain. This creates a delicate (and campaign-specific) balancing act. The PCs should gain a benefit from investing their goods, but that benefit shouldn't be level-inappropriate. If the investment benefits accrue too quickly, it is easy for the PC's investment income to provide huge quantities of resources. If the benefits accrue too slowly, then the whole thing becomes a waste of time because - by the time investment bears fruit - the benefits are vastly outstripped by the resources available to the PCs by virtue of being a higher level.
-KS
provides a means for investment in the campaign world. Essentially,