There are three kinds of funds in the world that anyone cares about, each with their own economy. Gold, Iliaster, and Mana.
GOLD
What is Gold: Gold is pretty and shiny and people naturally like it. Peasants have agreed to trade things for it on the assumption that that agreement will still hold true tomorrow.
The Gold Economy: Everything peasants can make or do is a part of the gold economy. Adventurers should be a part of it for the D&D equivalent of levels 1-4. PC's here track how much gold they have and if they buy a sword they mark off the 15 gold that it costs. PC's in this economy take the gold they are paid and buy better swords, horses, and gear. But gold can't buy magic items in this system so if the PC's come into truly fantastic wealth it doesn't break the game because once they have the sword they want and the horse they want they can really only spend the rest on swag and that's actually good for the game. By the time your PC's are 8th level it will be assumed that they have infinity dollars and have all the Castles and mink coats that could be purchased from people who value gold. The nice thing about this is that around that level you can do the "Big Score" plotline where the PC's finally kill a Dragon, hit the big leagues, and have a billion gold to cart home. Everyone feels like a big success and the game considers that right on schedule.
ILIASTER
What is Iliaster: Iliaster is crystallized magic energy. It naturally looks a lot like uncut diamonds and has a weak magic aura. Iliaster is required to make magic items. The process of creating a Flaming Sword requires one Iliaster crystal to do it. Bigger items require more crystals. Shattering or draining magical items will net you half the Iliaster it took to make them rounded up. Iliaster is also used to form magical architecture but that crafting process takes much longer and once the Iliaster is used there it's gone.
The Iliaster Economy: Iliaster is the trade good of the mid levels because it makes you better. It can be made by high level spells or rituals but the people who can do those things won't care about flaming swords anymore so for most mid level adventurers Iliaster is very much worth fighting for. You take magic items off your enemies and get the sweet bling inside so you can eventually turn it into Helms of Telepathy. Iliaster wouldn't have a set gold value because there would be no stable market for it as basically anytime you trade Iliaster for gold you are trading a piece of real ultimate power for beer money. An Iliaster piece might be roughly valued at 1000 gold pieces but that's for when some Peasant finds some crystals over an eaten adventurers corpse. An actual Adventurer in this economy works only for Iliaster because every piece he gets gains him a little more vertical power. The Iliaster economy will end once PC's hit the D&D equivalent of the 13th to 16th level range. At this point they will just be able to make their own Iliaster and will be assumed to be decked out in whatever magic gear pleases them.
MANA
What is Mana: Mana is soul energy and it's as much something you have as something you are. Powerful creatures gain Mana as a force wrapped around their soul. Anyone with a Mana score of 1 or higher is immune to natural aging and natural disease. Mana is generally immaterial in nature and has no natural physical form.
The Mana Economy: Mana can be given or taken from someone with a Mana score. Giving it is as easy as willing it to be given and taking it requires killing the person who has it. Mana can be expended in the creation of artifacts (in a system that doesn't exist yet) or the creation of other dimensions as if by a permanent casting of create demiplane. 10 Mana (about enough to make one artifact) can also be turned into a physical object called a Mana Diamond used almost exclusively by Gods for the services of other Gods, perhaps to secure century long pacts between rival deities. Mana score would gradually increase very slowly in beings that had it over the centuries but that wouldn't really impact a given campaign.
So for each stretch of levels there would be a resource the party was gathering and a resource they greatly desired. The parties resources would look something like this.
- Levels 1-4: Gold
Levels 5-8: Iliaster and Lots of Gold
Levels 9-12: Lots of Iliaster and Infinite Gold
Levels 13-16: Mana and Infinite Iliaster
Levels 17-20: Lots of Mana and Infinite Everything Else
Gold: Mundane gear and weapons, construction, and peasant work. services from low level PC's/NPC'S/Adventurers.
Iliaster: Minor and Major Magic gear and weapons, Magic Architecture, services from Mid level PC's/NPC's/Adventurers.
Mana: Artifacts and Artifact Weapons, Planes, services from from high level adventurers, Gods, and Outsiders because that's probably the only people that have it.