Well, I'm not sure where to go from here. Part of me is curious how to make this stuff matter in a game, so it's not just (as some have suggested) mental masturbation. I'm not talking costs or wealth tracking, but making it influence actual adventuring.
For instance, my group was delighted recently when our DM ran an adventure based on mining. ...
I think you've answered your own question, in a way. If you create a more reasonable economy, then the economy itself will suggest adventures and plots. For instance:
If land really is the source of wealth, then the control of deeds and/or titles will become very important. This can lead to adventures like recovering a deed, investigating who is working to keep a deed/title from transferring smoothly, or completing some task on behalf of a particular claimant so that they can one-up the competition and get the prize.
That cargo which has to get somewhere might not be a wagon full of gold coins. It might be the seed needed to sow the fields, it might be tools needed to work the land, it could be the grain, cheese, and meat being paid in taxes so that someone doesn't lose their holdings. When its lives and futures on the line instead of just an economic setback for the merchant, it makes the adventure more intense. There's guarding the goods or recovering them after theft. There's also thwarting or investigating someone who is trying to swap the good for inferior ones and so ruin the landowner.
Dealing with treasure also changes. In the default D&D WalMart economy, you can buy and sell whatever you want whenever you want wherever you want. If you go with something more historical, you have a totally different treasure distribution. Instead of just selling things, you're trading it for things, giving it as gifts for good will or future favors, and (quite probably) paying bunches in taxes. Frankly, I've had players have as much fun or more doling out the treasure and deciding who gets what and what gets hoarded as they did killing the monsters to get it in the first place.
You also add a whole level of favor exchange. The lord whose villages are being raided by an ogre and needs the PCs to deal with it might not have that handy bag of gold laying around. The PCs might be trading favors, might be granted land or a pass from taxes, or might be doing it because in the scheme of things they are obligated to based on their station or the order they belong to.
I also think that having a more robust economy also makes the treasure more interesting. I've never liked just having piles of coins except where the coins made sense to the situation. My players are used to getting things like furs, gems, artworks, tapestries and carpets, bags of grain, tools, weapons, wagons and tack, animals, utensils, and whatever else I can think of. It's not as simple as just handing out 500 gold, but for some of us it is more fun.
The trick that I've found ultimately is making the economy robust enough to be interesting but not so complex as to bog down the players with required reading. If you can't summarize their expected interactions with the market in a couple of pages or less, they're probably not going to remember it. (But that pretty much describes players and about anything in the campaign world.)
