When the shit really hits the fan, bullets and cigarettes are the only currencies.
Probably because Alcohol and Food can be fairly easily self made (germany, tradition of beer brewing, you'll find someone, also lots of farmes, even today)
On the other hand, tabacco is from overseas and requires some difficult manufacturing. Same for bullets, mostly.
I think alcohol and food would stay cheap. The things that would replace money are things you absolutely need but are hard to produce without modern civilation. Ie, cigarettes, bullets, gasoline, rifles and toilet paper.
But I expect "X" to be pretty long and pretty bad. Like Mad Max 2 bad i.e. years and maybe half the population dead.