Simply acquire 200,000 gil in your wallet at the same time.
in the town of Troia there is a Pub which has a membership card that cost 100,000 gil. if you buy it and follow the dancers into their dressing room you will find a gil farmer augment which increase the gil drop with 50%. which will help to become rich. It will not really help towards this one since it cost so much to get but since it will be an advantage on your way to 500,000 gil. You might still want to buy it before reaching this high. There is a chest in troia castle treasury with 30,000 and another in castle of Eblan with 10,000 which should get you going towards that membership.