When you get to the point of where you obtain the Pork Bun Recipe you will be able to do this easy exploit for yen. Pork Bun is made by two ingredients (Rice and Meat) which buying 1 each cost in total 125 yen (25 for rice and 100 for meat). Once you craft the pork bun it could be sold for a total of 250 yen (per bun) so you basically doubled your profit since crafting a pork bun requires only 1 meat and rice.

The only problem with this exploit is that pork buns are also used to increase your characters attack stat...but once you make a lot of yen you can then do the attack stat exploit easier. Cheers and Have fun