Okay, I have tried to find resources on the Internet, but it seems pretty hopeless, so I posit this question to anyone with more experience:

How do you create a fantasy currency?

My world also has a healthy system of exchange, particularly among the poorer classes for produce - e.g. one person provides eggs in exchange for milk or some grain. I do know that currency became more popular as trade increased, and wealth can be based on property of animals, such as cattle.

This world is used in a quartet of semi-standalone novels (i.e. you can read each one individually, but they are interlinked by character connections, setting and time period). In one novel in particular, there's a lot of travelling and, thus, the necessity of buying food and paying for inns. While my characters earn some money on the road, I really need to have a decent system in place for when I do the redraft and have to get this sort of thing accurate. A currency system that made too little sense would drive me insane.

So, any helpful hints, tips and links would be extremely useful. My plan is to use gold, silver, brass or something similar for coins (not copper - it's more valuable for pipes and stuff).

Thanks in advance.