When you get to exploring the Maze from Hell, the SW of the Isle of Fire, and the seas around the south of the Isle of Fire, the monsters are worth so much that the party members are going up levels like a monkey up a pole. By the time you reach the end-game, you will in all probability have run out of money. I remember duplicating Platinum Plate Armour and selling it to see if I could get my characters properly promoted. It's worse in WoX. In some places in there, IIRC you get the option to pick gold, gems, or experience as rewards, and if you pick XP, you can run out cash comparatively early. Thing is, you need the levels, because there's a monster in there called the Phase Dragon, and boy oh boy, just is he a handful! I think I have once managed to kill him without losing any of my characters.
I've found two of them in my sorcerer's backpack.