Just Coins

3 types of intercraftables coins and a coin sack, with no means to obtain in survival.
Coins have to be spawned by an admin or your favourite mod/plugin first.

1.2+ you can use /coin command.

Coin equivalences

1 gold = 8 silver = 64 copper
Coin sack = 9 gold coins

Craftings have no pattern, so it doesn't matter which slot is left empty.

To craft back to the previous one just drop one in a crafting, like this: