I'd recommend anyone interested check out the Noble House rules from the Song of Ice and Fire RPG by Green Ronin. It's simpler than the BECMI/Rules Cyclopedia rules for domains, but it's really good. I managed to rip out the Noble House rules and adapt them to D&D very, very easily.
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.
The other thing with economics is exactly what was commented earlier, PoL really wouldn't HAVE any. In a world where it is so treacherous to just travel to the next city that you have to hire professional adventurers to escort every caravan and STILL probably lose half of them there will be basically no trade, and thus really no currency either. Everything is like dark ages Europe, social obligations, custom, barter, real property, and the strength of the sword (etc). Gold is an abstraction that lets the game work in a reasonable way. In that kind of environment you'd much more likely have titles, rents, offices, favors, and maybe buildings and land, but you'd be lucky to ever see actual coins and they'd be of marginal value (can't eat them and there's no luxury you can spend them on with no trade).
Obviously a different type of setting is different, but it seems impossible to generalize. At some point the stacks of coins turn into big numbers across a level range of 30 levels from tough professional fighter to nascent demi-god. You can flatten the numbers down to a shallower curve, but that just means the DM's treasure allocation mistakes haunt you for a lot longer. Sure, 1 million gp seems silly, but OTOH it works and presumably when you get to playing with that kind of money you're not buying and selling in Podunkia anymore. Millions of GP actually DO make sense for instance in Eberron.