(Psi)SeveredHead
Adventurer
As a former d20 Modern GM, I have to say ... do not do this!d20 Modern had an interesting rule for purchasing things. Rather than keeping track of how many dollars your character had, instead you had a "Wealth Score." If I recall correctly, depending on your score you could buy certain classes of items at no cost, while others you had to roll for, using your Wealth Score. If you rolled a certain amount, you got the item, but it lowered your Wealth Score.
For example, let's say my character wanted to buy a stapler. My Wealth Score would probably be high enough that my character could just buy one without worrying about it. But if my character wanted to buy a car, they would have to roll for it. Roll high enough, and I purchase the car, but my Wealth Score takes a hit (until I get paid for this next adventure, of course!), roll low and I fail to buy a car ("Your card is denied.").
What would this look like in 5e D&D?
I could see characters having a Wealth Score based on their Background (Noble = high Wealth Score, Urchin = low Wealth Score). Maybe something like a +1 to +5? And you would add in your Proficiency Bonus.
Your Wealth Score Bonus would allow you to purchase items of a certain value without spending any Gold Pieces. For example, a Wealth Score of 5, for example, may allow you to purchase items of, let's say, 2 Gold Pieces and below without spending Gold Pieces.
A character can raise their Wealth Score by investing money. Maybe each level of bonus would cost 1,000 x Bonus GP. So for example, to raise your Wealth Bonus from a 3 to a 4 would cost 4,000 GP. From a 19 to a 20 would cost 20,000 GP.
As Wealth Bonuses increase, it could unlock certain features of the game for characters. For example, you might need a certain Wealth Bonus in order to purchase or upgrade a keep, raise an army, or buy magic items. Certain areas of cities or kingdoms might be closed off to characters with low Wealth Bonuses (but open up for a high Deception check!). Fun powers could include the ability to always have a fresh horse ready at every city, be always dressed in Fine Clothes, earn a Reputation, and so on.
Well, these are just some thoughts. How would you do a Wealth Score in 5e? What would it be used for?
D20 Modern didn't have "lifestyle costs" so nobody had to buy anything. My first PC was a martial artist who only ever bought two things: a leather jacket, and a "magic stick" that someone conned him into buying. (Many PCs ran out of money at character generation as they bought guns and armor, but afterward had few costs so in a few levels they were wealthy again. By contrast, I've seen crafting PCs use up Wealth very quickly.) If you had a Wealth score over 15, nothing with a lower cost ever cost you money. Unlimited cheeseburgers!
The game had a Profession skill that I came to hate. It wasn't Profession (career) but just Profession, that gave you money every level. If your character was a mercenary, this actively hampered roleplay.
You could split the economy into two (so one way of spending cash and another to buy magic items) but I think a lifestyle cost section would work better for the "cash" part. Any PC who is going to adventure while investing should hire a steward to take care of that for them. Said steward needs a personality, might be targeted by enemies or become untrustworthy, etc.