Infinite Money

Once you have beat the game you'll get the chance to buy things like weapons from the store. You make money by doing things like finding treasures etc.

Once you do something that gives you a large amount of money (like Master Ninja for 25k). Save your game (if it's not at an auto save area) and then quit back to the main screen. Then load up the game again you'll keep getting the reward each time you do it. Soon enough you'll have enough money to buy everything without having to find all the treasures.