Pre-war Money has a base value of 10 caps in Fallout 3, and a weight of 0. Obviously, it's a good thing to collect and trade in for caps. Are there any NPCs who value Pre-war Money more than the average merchant? Or, really, is there any use for Pre-war Money besides just selling it to a regular merchant?
It is a good 'secondary currency' along with Cigarettes. Since vendors run out of caps easily, it is sometimes useful to have something you can convert your heavier items into for transport until you meet another vendor, or they get more caps. Prewar money works quite well for this, though due to the buying and selling you won't get full value.