Here is what you should do (you should have beaten the Elite Four).
1) Go to 5 Island's Resort Gorgeous.
2) Make your first Pokemon hold an Amulet Coin.
3) Use your VS seeker on 2 ladies named Lady Jacki and Lady Gillian ( their Pokemons are level 48-50 ).
4) You should get in all 39,000 dollars.
5) Run around a bit and repeat.
If you use a Pokemon with the move Pay Day, your rewards will increase in the battles. Carry 5 Meowths with the ability Pickup to get useless items while running, to sell and speed up the process.