If you check a certain bush on Artistry Highway in the base game, near the train tracks before the forest area, an Egg will pop out. It didn't have any use besides being a minor healing item (and a Paper Mario reference). So in the DLC, I decided to give it a new use! If you use the Egg in battle against a Karma Chameleon or a Pile o' Gold, it will turn into a Golden Egg that can be sold for a high price.
The game is full of tiny little secrets like that! :P