While different types of currency vary, the value of currency is the same for any coin minted by a body. Unminted coins - which are basically discs of metal with no markings - are worth 1/2 of the value of minted coins. The coins of Kyujitai, Githas and Zakhara are very different from those of Leng, and must be exchanged. Within Leng, whether a coin has retains its value in a foreign country depends on what currencies are in standard use there. You can use a Vintish Crown in Lorknir, but in Arelon it will exchange only for half value (5 sp). Large cities will often have money-changers that can change your currency at much more equitable rates, usually only taking a cut of 5% to 20%.
Any coin that is minted will usually bear very clear signs as to its origins - most coins minted in Leng will bear the name of the kingdom, its ruler and the date that it was minted. Leng coins are generally quite small, being about half the size and weight of a £1 coin. The weight of a coin is usually around 0.01 pounds, meaning that every 100 coins you carry weighs 1 pound. The volume of a coin is about 1 cubic centimeter.