The changelings didn't just siphon off emotion from people for food—their entire economic system was built around it. It was an invisible, intangible currency that could only really be manipulated, stored, and divided by changeling magic. A currency that was consumable, in constant demand, and constant need to be replaced, as necessary for getting up in the morning as it was to paying your taxes. He was only even aware of it because of his auspex. No wonder they didn't care about their own gold coinage.