[@Verse Zero]I'm not sure because I'm neither an expert about trading noble metals nor about medieval currencies, but from the strict mathematical point of view I'd guess that it should be 100 copper pennies for a half moon instead of 100 copper for a moon. Otherwise one could still trade 200 pennies for two moons for a sun and trade that back to 400 pennies.
[quote]Copper Penny: 100 pennies make a Moon and 400 makes a Sun.[/quote]

Got some idea for a human character in mind :)