If this isn't nice, what is?
I suppose the coin supply could run out, although it seems unlikely. One possible house rule would be to mimic the "out of stones" rule: If a player is entitled to receive a stone that is no longer available in the storage, all players must return half the amount (rounded down) of their stones of this size to the storage. Then the player receives his stone(s). So, if no coins are in the supply, all players would return half the amount of their coins (rounded down) to the supply.
The rules don't state to do that with coins. My players have run out of coins many times so far. We've been playing where they simply don't get to take more money at that point. It's very easy to run out of the coin supply.