Aw, I was hoping for something closer to Tolkien's mythos. Something like:
1 gold coin = 20 silver pennies
1 silver penny = 12 copper coins
1 gold coin = 240 copper coins
Well, there isn't really anything canon about exchange rates in Tolkien's works, excluding the coinage system in Gondor (castar, tharni, etc.) from
The Peoples of Middle-earth, which we didn't use anyway.
We only know that Bill the pony was bought from Bill Ferny for 12 silver pennies and that it was considered three times its price, but even then, was it because ponies usually cost 4 silver pennies, or because Bill looked so old and starved that it was worth one-third of a normal pony? (eventually, we went with the second hypothesis, a bit more extravagant, but otherwise copper coins would be far more used than any other currency)
So, in the end, ease of conversion took precedence over any conjectures.
It is not our part to master all the tides of the world, but to do what is in us for the succour of those years wherein we are set, uprooting the evil in the fields that we know, so that those who live after may have clean earth to till.