So, I studied the DMG and XGE for the Magic Item Prices to make my own List and I also looked when according to the DMG and XGE Magic Items are rewarded to the players.
So I changed the Item Prices by rarity for my campaign with the goal, that the players will find magic items that they can't afford yet to make the found ones more special:
So the new maximum prices are:
Common 100 (DMG 100, XGE 70)
Uncommon 900 (DMG 500, XGE 600)
Rare 25 000 (DMG 5000, XGE 20 000)
Very Rare 200 000 (50 000, XGE 50 000)
Legendary 1 250 000 (DMG 500 000, XGE 300 000)
Now, I'm not taking the maximum price for every magic item, but for all magic items that I think can change the power balance of the encounters (+X weapons and armor, items that give you the abillity to fly ect.pp.).
So, if you take the approach that magic items you can buy are no more "you pick" than the ones you find, then the gold buying magic items just produces more plot.
Finding someone to sell you, or make, a legendary X is an adventure in itself. The reward? You spend money and get the legendary X.
And maybe the spending of the money isn't "hand it over", but "I want Y", and you have to spend money to get Y, then deliver it to the person selling the legendary item.
I will agree that there is little direct support for effectively leveraging your gold.
A lot of the options in the DMG just amounted to ways to waste gold, not to use money to make money. I will second though that Strongholds and Followers does a decent job setting up what you are talking about, and the second book, Kingdoms and Warfare is getting towards the end of play tests and writing, and will be available soon, which gives you actual kingdom management stuff. Nothing nitty-gritty or detailed, but the gaps should be easy enough to fill in if you wanted to.
Spending money to make money should be spending money to make adventure.
As an example, sailing ships used to be huge gambles. You bought it, and there was a decent chance it wouldn't come back from the trading journey. Your trading post gets threatened by orcs. The keep you made is on claimed land (by more than 1 party), and one or more of them demands fealty.
The engine shouldn't be "spend gold, get income". It should be "invest gold, get adventure".
That adventure in turn can generate gold.
Is this a bit of an naughty word thing to do as a DM? A goal is to turn the player's actions into adventure. And a safe ROI isn't adventure, nor does it match historical investment.
Investments that safely return even 1% per year don't exist prior to an industrial revolution by a dominant superpower. Because 1% per year ROI starting 4000 years ago is x192,972,369,947,315,104 - 1/5th of a billion billion.
Instead, the economy looks more like:
1) Hording wealth is expensive because you have to defend it.
2) You invest wealth in order to make it harder to steal.
3) If you invest 100 tonnes of gold now, and in 20 years you can extract 50 tonnes of gold from it, and it kept itself safe, score.
The rich where the people who owned land and had some kind of security that they could keep it. They spent their resources maintaining that security through any way they could. Produced wealth was spent soon after it was produced, not horded; first spent on attempting to maintain the land you had, and then spent on more security.
If you got extra security (troops) you spent it on attacking your neighbors to take their land before your neighbors did it to you. Eventually larger states formed, and you cooperated with your neighbors to attack people even further away. This also kept your peasant population down as it would quickly outpace the requirements to farm your land; having some die in wars means less mouths to feed.
Cities where dumping grounds for excess population, and filthy and disease ridden. They would generate wealth in easier to trade forms, so where valuable, but where generally population-sinks not sources; more people moved into them than moved out, and more people moved in than they grew. It was better than starving in the fields -- think of it as an alternative to shipping people off to die in wars. You send them to a city, where their death rates are similar, but you get wealth without
it being quite as zero-sum.
It is only on the margins that investment opportunities that generate net profit come out, beyond the "produce food" -- almost all of the wealth is literally "food". That food gets horded by peasants who don't like starving, so isn't cheap to get even if you own the land, and they use the food to produce more peasants, which you as a ruler don't have a need for ... so you spend them in a city or in wars for stuff you might find useful, like more land or trade goods.
But that is just blather.
The main point is, you can frame it as spending money to make money for the PCs.
But the mechanics should be investing money to generate adventuring opportunities. And change the world.
I can agree that rarity shouldn't be the standard, but it is, and it is supposed to line up with power.
For price? Naw, rarity isn't all that bad.
And yes, this means that item X which is worse than Y is cheaper. That only really matters if you have a magic mart
My bigger complaint is that pricing an entire category with that huge of a gap, is insane. Every other tier of item, combined, can fit inside that gap. It is FAR too big.
I don't see why.
How much gold the PCs has varies, and the amount of gold you as a DM feel like you should be sucking out of them or giving to them when they sell stuff varies.
The idea of that range is to give the DM a ballpark of "lots" of money if you follow something vaguely like the DM treasure schedule. That really is all that is needed, for a DM, at least one not looking to have a magic mart.
For a player looking to magic mart, it is useless.