Even though I'm a quasi-prepper, I haven't given this much thought recently. No significant target is to the windward side of my city for hundreds of miles so the risk of fallout is minimal. I'd head to Kroger's/Walmart and fill my car with nonperishable foods, salt, multivitamins, TP, etc. I'd buy way more tradeable goods than I needed... salt, rice, dried beans, TP. Many post Apocalypse books gloss over how many calories you need to survive for a year. It's a LOT, especially if you take in survivors with nothing but the clothes on their backs.
Then, I'd go to the nearest gun stores and buy 10,000 rounds of 5.56 and 9mm ammo... again, that's tradeable. After dumping that stuff at home, I'd buy a dozen gas cans and fill them. Again, tradeable.
The cost of these steps would be minimal. The food could be used over time or donated after the crisis is over. The ammo will hold its value for many years.
If you live in a big city, you'd better have a bug-out bag and bug-out plan. Many city dwellers have no plan and would just hunker down until help arrives, but if the grid goes down, help won't be coming. Looting and burning will start within 24 hours and when food runs out, gangs will go from house to house taking what they want. You need to be gone before that happens.