When the bombs fell and the chaos ensued, most of the money was destroyed in the blasts and fire. And even if it wasn't, the inflation was so high, it would be practically useless. For this reason, the main way of trading in the wastelands is material exchange. The price depends on how well both parties haggle and how valuable a certain item is in the wastelands.

However, not everyone wants everything, so offering a lonely traveler a cow for his rifle will only end in him giving you a strange "are-you-nuts?" look. More exotic things, like salt, fire bee spores and similar will fetch a higher value.