A quick way to triple your current cash is to do the following:
NOTE: These can only be done after reaching Jinga and unlocking the Secret Market from Kenjiro!
1) Go to the secret market with at least 12000G and talk to the peddler on the far right.
2) Choose "Buy Expensive Materials" and scroll down until you reach Earth Dragon Wing.
3) Buy as many as you can (12000G is minimum, bring as much as possible) and head to Jinga
4) Run up one screen and go past the Save Point. Head left after passing it and climb the set of stairs you find. Enter the house on the very top and talk to the man on the right.
5) Trade ALL of your Earth Dragon Wings for Sun Faulds and Exit Jinga.
6) Now go to any shop and sell ALL the Sun Faulds for a whopping 32500G, almost triple what you bought a single Earth Dragon Wing for!