In my current game, the apocalypse was due to a more-or-less unsuccessful alien invasion that still managed to screw large portions of the environment and infrastructure. In discussion, it was decided that we were going for a fairly classic 'Mad Max' aesthetic, but that the players weren't actually that interested in basic material shortages like food and water (they were much more interested in shortages of things like trust, love, and security), so the game occurs in a valley with water, surrounded by hills, in the middle of an huge desert, but with several different hardholds and gangs in the valley.
So what the basic person does varies wildly depending on what settlement they're from and what its industries are:
In the trading town/gateway to the valley, many are merchants, some scavenge the desert, while a few do basic farming and a fair number of others refine oil into gasoline.
In the town made in a giant alien fungus everyone works with fungus, farming the edible stuff, or using the stuff that extracts oil from the ground to do that (for shipping to the people who refine it).
In the other major farming town, people mostly do more conventional farming, plus animal husbandry (of the fanged chickens, mutant cows, and spider-goats...the last also provide spider silk, for use in textiles).
In the underground bunker/town started by survivalists, they mostly make ammunition (they have the facilities for that, there) and trade the ammunition and occasionally bits from their supply of old-world stuff for the food they need.
In the town built into the crashed alien space ship people mostly harvest the various bits of the ship (particularly the various liquids the ship produces, many of which are potent drugs of one sort or another...the glowing green stuff is reliable birth control, while the pink stuff is basically cocaine + PCP, and so on), or organize the above efforts, and some do a bit of basic farming as well.
In the PC Maestro D's place (which is separate from any of these, almost a mini-hardhold in its way), the people make their living providing music, food, and/or sexual services to travelers between the various communities already mentioned, plus all the mini communities that exist between them.
All these communities also have gangs/militaries who receive money for their work or raid those who lack their organization/firepower.
So...yeah. That inevitably varies quite a bit from place to place. In all of them I agree that it's mostly scraping a few barter together a month, but probably more like 2 than 1 most times and places. In the course of a year they can afford a new set of clothes, some livestock, some tools, and the occasional small 'extravagance'. But it'd definitely be less in an area where food and water are more scarce.