I've been puzzling over this one for a while. The standard D&D money system with set price lists really doesn't feel right for a post-apocalyptic setting. A good post-apocalyptic money (or barter or whatever) system should have two qualities:
First, there should be no set price list, because prices are going to vary dramatically in a world where trade isn't safe or regular, and shortages are frequent. Also, I'm constantly going to be introducing new things for sale and I'd rather not obsess over finding the "right" price for it.
Second, whatever people use as money needs to have clear practical value as something other than money. No fiat currency, obviously, but also no shell money. Ain't no patience for fancy money in the wasteland!
So that leaves three options that I can see, and I'll show you what I settled on in the end, including how I set prices.
Alright, So Actual Money Then
Basic low-tech gear, minor luxuries like wine, and low-tech weapons and armor cost scrap. By low-tech, I mean anything a pre-industrial culture could make, not withstanding that more advanced alloys may be used since there's pre-tech scrap metal lying around.
More specialized gear, small animals like cats, dogs and chickens, low-tech vehicles like bicycles, rowboats or wagons, and modern tech weapons and armor cost silver.
Land (in town– land in the middle of nowhere is free to whoever can hold it), cobbled-together modern tech vehicles (motorcycles and small buggies), and small futuristic tech devices, weapons and armor cost gold, as do really fancy luxuries
Magic items and small futuristic vehicles cost platinum
Move up a coin size if it’s particularly fancy, i.e. fine food rather than cheap food, a private room in the inn rather than the common room, a custom car rather than a beater, a decent sword rather than a spear or dagger, chainmail rather than padded. Move up two coin types if it’s really fancy, i.e. plate armor, a rare vintage of wine, or a bazooka.
Move up a coin type for every order of magnitude of size above personal items– car, house, ship, fortress, etc.
Move down a coin size if it's a piece of shit– a care that might break down any time now, spoiled food, a rusty weapon, etc.
Items cost d4 coins. If size is relevant like with armor or meals, d4+1 if sized for someone big, d3 if sized for someone small. Double, triple or quadruple the price if there’s a shortage.
If there's a glut of an item, you might get a small discount on one, but you can get much bigger discounts for buying in bulk. In other words, if there's a surplus of something it's meant for export to other towns.
Plate armor: low-tech armor is scrap, but custom-made plate armor is extremely fancy and expensive, so that's gold. If you can't afford that, mass-produced 3/4 plate is silver.
A modern sniper rifle– modern weapons are silver, but sniper rifles are kinda fancy, so gold. If you can't afford that, a lower-quality bolt-action hunting rifle could be silver.
A crappy modern-tech motorcycle would cost gold. A big all-terrain van that can hold the whole party plus hirelings, gear and loot would cost platinum, as would a really good motorcycle. A really good big ATV would cost tens of platinum! On the other hand, if you really want a ride for the whole party at cut-rate prices, you can get a sketchy battle van for gold. Your funeral.
Whatever price you roll, that's the price in that town for the near future. Yes, the same item might cost 4x as much in the next town over. If so, there's a reason why prices haven't equalized, like bandits preventing regular trade between the two towns, or the towns just don't get along. Figure out what the reason is, and turn it into an adventure seed.