Whenever you read a fantasy story, there's always some sort of cash involved. Characters purchase supplies or pay for a night at an inn and always drop a few coins into someone's outstretched hand. Some of those stories have markets where you can either buy or barter. But is it necessary to work up a monetary system?
Money is the all mighty motivator no matter if you are just living day to day in the real world or you're absorbed in reading about another world. In Harry Potter, the first book, I'm sure there was a collective sigh of relief when everyone discovered that Harry's parents and had left him a sizable amount of gold. Seriously, you didn't think his aunt and uncle would fork over anything for Harry, did you?
If you're a fantasy writer, you'll know the standard forms of acceptable payment is copper, silver, gold or platinum. Or all of the above, including brass. Copper is the cheapest. Think pennies. Silver is next, then gold and highest is platinum. Some fantasy worlds don't even deal with platinum or brass. They aren't "standard" forms of currency. Sometimes gems are involved, but how do you put a price on a gem? By size? Possibly. There are all sorts of types of gems, so you could base your monetary system around that.
Other worlds like to use bartering. But you can't really barter for everything. Sometimes a copper is needed.
No matter what, you can't get away from money. It is something we're all familiar with, it is something everyone wants more of whether they are real life or fictional. The more money, the more power. But what if more money didn't mean more power? What if it meant the opposite and you couldn't get rid of your money fast enough to save your life? Food for thought.
In my world, I have several types of currency as well as bartering engineered into my monetary structure. A Dract is gold, a Kemya is silver, a Barou is brass and an Aurn is copper. The Elves and Dwarves use gems. The highest being a sapphire, then an emerald and lastly an amethyst. One kingdom goes rogue in the currency department and their names for the gold, silver, brass and copper coins are Falah Tree, Herring, Sheaf and Gull. There's a whole story behind it, don't ask. Or ask later.
One island uses different gems and minerals for their monetary purposes. The highest is jade, then ivory and finally onyx.
Most of the time, I don't get too detailed in the information of what coinage or gems are being passed during a transaction. Mostly I refer to the money as coin or gems. The mention of the gold Dract is prevalent due to Farrehn and Vendras always betting on silly things. A gold Dract is always at stake. Also, gold is pretty easy to come by in my world, not everyone has it, but a very large percentage of the population does.