"Let's go with ogre eyes," Scrooge said. "Can't get as many off of an ogre as you can pegasus hair and pegasi can periodically be found in herds. So ogre eyes are rare and can't be duplicated easily. Makes them a good base for currency."

He held up a finger.

"Money - paper money in particular - is just paper. The only reason it has value is because everyone involved in trading for goods and services comes to an agreement on more or less what it's worth. It's a promise: 'This piece of paper is worth this amount of currency if you needed to trade for the real stuff'. Given its rarity, let's say that one ogre eye is worth one thousand pieces of specially-printed paper which are Witch Dollars. Following so far?"