Because it was so expensive, I only got him to sell me a barrel's worth.
Nevertheless, it still seemed to have cost 25 gold coins in total.
Among Elle-sensei's classes there were also lessons about money.
Based on my life experiences here, if I were to put it in terms of modern currency it would be something like『1 gold coin (100,000) = 10 silver coins = 100 large copper coins = 1,000 copper coins』.
Intuitively, 1 copper coin is worth 100 Yen.