Coinage - it's complicated

D&D coinage tends to be very simple.  1gp = 10 sp = 100 cp, with sometimes platinum and electrum thrown into the mix.

However, this is not very historically accurate.  Things were often much more muddled than that, and some games have attempted to emulate this.  Warhammer frpg for example uses a 1 crown = 20 shillings = 240 pennies, which does have some historical basis.

In my experience over the years, I've discovered that while a more accurate and complex coin system can be satisfying for the GM to research, design and use... unfortunately in actual gameplay, it's kind of a pain  (the above Warhammer example is about as complex as you want to get).  Your players probably won't like the bewildering array of coins and strange ratios, and it slows the game down.  

So what to do if you do want a complex coinage system anyway?  You use "a unit of account".   This is a unit of currency that is widely used to measure value, even though actual coins of that value may be rare, or non-existent even.  The actual specific coins used for this are not important, but the PCs know instantly what the value is.   This level of abstraction speeds up the game, and the various coinage can fade in the background *until* they becomes important because of a plot or logistical issue.

For example, a specific coin is being forged.  The PCs are investigating.  Or the PC found a fortune... in Oxide Ingot , the fortune weight tons, what to do?  

In my Yoon Suin campaign, the unit of account is the Rupee - a big fat silver coin that has a fair amount of prestige associated with it, but is rarely seen in circulation.   So if the party is offered a 10 000 rupee reward to undertake a dangerous mission, they will probably paid in a mixture of gold and silver pieces.

All the players need to know is 1 gp = 5 rupee = 10 sp = 320 cp 

The actual values:

1 mohur = 3 gp = 15 rupee

1 gp = 5 rupee = 10 sp

1 rupee = 2 silver pieces = 16 anna = 64 paise = 192 pie

1 sp = 8 anna = 32 paise = 96 pie

1 anna = 4 paise = 12 pie

1 paise = 3 pie

The coins

The Mohur is a large, rare gold coin minted in the Yellow city.  Being paid in actual Mohur is a privilege, and people prefer hoarding them than spending them, keeping them out of circulation

The "standard" gold piece is a small piece of gold, used by merchants and the wealthy, mostly minted in the oligarchies and the hundred kingdoms.  (3.84 g)

The rupee is a large silver coin, as noted above, and being paid in actual rupee has some prestige.  Like the Mohur, it is frequently hoarded.  A rupee/day is the "minimum wage" - what a free laborer can expect to get, although they probably will be paid in a mixture of silver pieces, annas and paises.  

The "standard" silver piece covers a large array of mediumish silver coins that have over the centuries being somewhat standardized to be worth half a rupee.  The half-rupee coin itself is uncommon but isn't seen as special.  A lot come from the hundred kingdoms, the oligarchies and the Mountains of the Moon.  These coins are in large circulations   5.4 g on average

The Anna is a very small silver piece, minted mostly in the Yellow City.  It's frequently debased and forged, and as such the anna is not accepted for large sums and is seen as suspicious.  "Being paid in annas" is a euphemism for doing unsavory deeds for money.  However there are many in circulation. (0.675 g)

The Paise is a large copper coin with a hole in the middle.  In the Yellow City, the majority (easily recognized due to the square hole) are imported by merchants from far away Xian.  This is a coin frequently used by the poor but unlike the Anna it is seen as "honest money" and is in very large circulation.

The Pie is a hardened lead coin of little value, seen as fit for children and beggars, and is minted in the Yellow city.  However it is rarely counterfeited (why bother?) and can still buy you a cup of tea, so it's worth something :)

Another form of "currency" is the tea brick.  A tea brick of "the third quality" (the most common) is worth 4 rupee and weights slightly over a pound.

Lastly, there are the is the silver and gold talents, an enormous mass of precious metal (26 kg!) that only the ultra wealthy deal with...  (1 gold talent = 6771 gp)    1 silver talent = 2 407 rupee, about 2 476 cm3)