There are plenty of people who want to see the reemergence of the old governments, but until that time, old money is pretty well useless. This is not helped by the fact there are some groups who actively don't want to see the nation return. (Notably NB believes "in god we trust" on bills was a bald faced lie, and the Free City of Tesla sees it as an anachronism)
A few groups have local currencies -
- NEST citizens trade energy credits and kilowatt hours for bringing the technological relics still available running.
- Vegas claims to honor US dollars, but in practice uses stamped pay-chits since its all to easy to rob a bank in the wasteland and ride in with millions.
- New Birmingham has a provisional currency as well, in theory based on hours of effort.
First and foremost, I advocate turning any shopping trip into a role-playing experience. The players should state what they want, and why, and really try to bargain. NPCs should give a counter, and if the argument gets a bit heated, so much the better. Remember that the black market doe not have fast food style menu-boards of prices and products.
I'm considering a couple of ways to explain value in the game. One is to run with the NB idea above - base things on a labor/added value economy. At its base, a few hours of work tilling fields or mending buildings earns two or three meals, and some shelter. More complex or difficult work is worth more, (ie fewer hours get the same entitlement to rest and food) - and from there we come up with an arbitrary number that equals one labor unit/minimum wage.
Another possibility would be to create some sort of matrix cross referencing settlement size versus level of desire. To an individual or small homestead a tractor represents too many hassles in the form of maintenance and fuel to be worth anything, while are larger town could really use one. Conversely a single person might need weapons for defense, but a group is more likely to already have these things.
In turn this lead to an idea based on Maslow's hierarchy of needs. The smaller the group, the lower on the pyramid trade needs to be. Individuals want safety and shelter, groups are focused on being comfortable and long term goals, while the city states are seeking machinery and old technology to become nations in their own right.
Ultimately, this should be up to the GM to help establish the mood of the zone. If weapons are at a premium, hen simply getting a gauss rifle or two is a reason to go on a dangerous mission into some ruins. Should the focus of the game be more on avoiding direct conflict and exploration, then perhaps weapons are not in much demand at all, while good quality maps and navigation systems are practically worth killing for.
I'll admit this is a temporary solution, and I'll need a more established price system eventually. However, I'd feel better about its design with a bit of feedback. Its a pretty tall order to figure out the value of things in 2050, and then factor in the changes since then.