When Foundry Inspector is on the battlefield and Treasure Keeper is put into the graveyard, would Panharmonicon be able to be cast because artifacts cost one less to cast? Or because the CMC is originally 4, Panharmonicon wouldn't be able to be cast?
The CMC of a card is always what is printed in the top right corner of the card, so Panharmonicon always has a CMC of 4
So no it can't be cast off of the dies trigger of Treasure Keeper.
So no it can't be cast off of the dies trigger of Treasure Keeper.