Currency (Fallout Supplement)
From D&D Wiki
Depending upon the region of the wasteland many of its inhabitants utilize several forms of alternate currency since the American Dollar is not even worth the paper it is printed on.
The common currency in the Fallout Universe is bottle caps. Bottle Caps are often found hoarded away, from the corpses of slain humanoid enemies, and taken from common non-alcoholic beverages. Many wasteland survivors take care to save up their Bottle Caps for something they desire.