Regardless of the rule system I use for my fantasy campaign I nearly always jettison the coinage system in favor of my own. It is nearly a direct copy of what Harn uses.
1 silver penny = 250 to 1 lb (a US dime sized coin)
1 gold crown = 320 silver penny = 16 to 1 lb (a US half dollar sized coin)
The following in theory "exists" but I don't use that often
1 silver mark = 240 silver penny = 1 1b (rounded it is really like .9 lbs) (this is a silver bar with a maker's mark stamped on it)
1 gold penny = 20 silver penny = 250 to 1 lb
Note that the pennies should be 256 to a lb but are round to make calculation easier. I handwave it as they are slightly debased compared to a 1lb of pure metal.
What I found that a system where you have a one common coin and one coin that is really large in relative value works better as far as giving the sense that treasure is valuable. The hordes of silver pennies are appreciated but it is the rare finds of gold crowns that make the players go oooo.
If you read anything on ancient coinage you will find trying to make heads and tails of it complicated. But as far as our game goes there are some basic principles you can use to make a coinage system of any arbitrary complexity.
The first thing is to remember that everything is based on weight. So your pennies can be 1/100 of a lb, your denarius 1/16 of a pound, and your groats 1/10 lb. The conversion factors flow from comparing the weights of the different coins. You may want to consider using drams instead of fractions of a lb. There are 256 drams in a 1 lb.
The second thing is the ratio of the value of silver to gold, along with the ratios of any base metals you are using (copper, bronze, etc). Historically Silver to Gold was about 20 to 1.
By knowing the weight of the coin, and the ratio of value from silver to gold. You can create a system of arbitrary complexity. I recommend that if you want to create a medieval feel to your coinage that you fractions of 12 or 16 instead of a decimal system.
Finally if you want to include debased currency just note the percentage loss. For example a silver penny from the Kingdom of Bythinia is worth 50% of a silver penny from the Empire of Argentum who still uses the full amount of silver in their coin.
Debased currency is the prime reason why trying to study anicent currency is so hard. One king may try to strike 20 drachmas from a lb of silver while a later king strikes 24, and further on the king starts striking 28 drachmas from a lb of silver. In each case mixing in base metals to keep the weight the same. Eventually the populace starts to wise up and begins to treat the new issues at a lesser value. Even they don't notice inflation will eventually kick causing prices to rise.
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