The gentleman on the pokemansion route with vs seeker + amulet coin gives me insane money. All he has is a singular chatot
commented 52 minutes ago by ~Smoothie~
The very first time when you battle Gentleman Jeremy in the route where Pokemon Mansion is, he only has a Level 19 Chatot. On beating him, you get P8,056 (with the Amulet Coin). It's more recommended to money-grind after completing the Elite Four, because the number of times you battle certain trainers, not only their Pokemon's level increase, so does their money.
For example, when you challenge the Gentleman for the second time post-game, his Chatot is level 31 and the payout increases to P12,896. The next time the Chatot is at level 40, but at its maximum cap, Level 60 Chatot with the prize money being a whooping P24,480! Remember that you need to be in the post-game and completed the Elite Four so that their levels and prize money increase consecutively the number of times you battle.
Near him, Madam Reina has a Level 19 (or 60 at maximum cap) Roselia. She also offers the same prize money. If you beat both of them, you can reap P48,960 from both of them combined, assuming you're in the post-game with the Amulet Coin equipped.
Rich Boy Jason and Lady Mellisa on the same route outside of the Pokemon Mansion have a Level 19 Luxio and Buneary. The latter can have increased money, to a maximum of P19,680. This is lesser than the above, but it's decent enough.
tl;dr Gentleman Jeremy and Madam Reina, in the post-game with Amulet Coin and max level cap can give P48,690 per try, which makes it very efficent and fast way for grinding coins.
Also, remember to turn off Battle Effects in Settings to save more time.
Source: This video was very useful