I think the fun of this system would be working out the value of each item in relation to each other item, instead of having any set item be a currency.
Mostly because each individual persons needs will be different. Somebody without items that have positive quench value will value water, drinks, and containers more than somebody who has tons of water. Somebody with who had a gun will value ammunition and magazines for that gun more than other items, especially if they are running out (assuming NPC’s don’t just magic items out of the ether when they run out). Somebody who is hungry and has no food will value food more. Craftsmen will value raw components more than normal (to a reasonable degree, they still want to make a profit on their labour obviously).
Although each item will need an internal and hidden value as compared to every other item for some sanity to be involved. That value could be determined at the start of the game by pre cataclysm values, and perhaps the item type could have a value curb that changes over the course of a few game years. Items that can no longer be produced, except by factions, could go up over time. Some notable items would be gunpowder, plutonium, and luxury alchohol. Other items could drop in value over time as well, like raw food or easy to create food. Of course, this would be ruled by their usefulness or enjoyment value. Stuff like multivitamins and antibiotics, despite being technology that might very well be lost to the ages, may not be valuable because they would be to old to be effective after a year or so.