Sorry. Not a good title, but my point is - All sorts of coins and artifects from all over the world are found where they originally shouldn't be, so it means it was brought there by a trader or a merchant. My question is - if a Roman trader came to your village somewhere in the Baltics, offered a coin (gold, silver, whatever material) for your goods, then how do you decide how much is the coin worth? What do you do with the foreign coin as it holds no material value?
This thread might help a bit! Not sure if it will answer it all though. http://www.reddit.com/r/AskHistorians/comments/10bp92/how_did_prescientifically_advanced_civilizations/?
The coin was generally just a set weight of some precious metal and that was its value. Stamping a coin (giving it some pattern or image) simply served to identify that it was a certain weight of gold. If a coin weighed, let's say, 35 grams and was gold, the pattern or imaged stamped would just make it easy for people to know that it was 35 grams (or rather the equivalent weight in whatever system was used) of gold. To know how much a foreign coin is worth, you would simply weigh it.