Go to the 3 star restaurant and equip the Pokemon in front with the Amulet Coin. All the Balm Mushrooms you get will be for profit, easily making you rich.
The Trainers at the Battle Chateau at first will pay out 1,000 per battle. However, as you get better ranks it can end up over 20,000 per battle.
Use an Amulet Coin and/or the Prize Money O-Power in these battles to make a large amount of money.
After accessing both the north and south parts of Lumiose City, travel along Autumnal Avenue and enter the building that has a black sign with two stars. Accept the rotation battle request from the waiter. You can earn Big Mushroom items, depending on your performance. Each of them can be sold for 2,500. Repeat the battle as often as desired. Note: You will no longer have access to this after you earn the Voltage badge and leave the city.