OK... In the beginning of the game there is a boy standing near the Petalburg Woods. Battle him. He will have a Zigzagoon. But when you finish the game and get the poke'nav, he will want to battle you again. Battle him until his Zigzagoon evolves into a Linoone. It has to be about level 30-35. This is what you will need.

A pokemon team
A pokemon that knows the move Thief; It must not be holding anything.
A pokemon that is holding Amulet Coin.

First use Thief to steal Linoone's nugget. You can sell it for 5000$
Then switch to the pokemon that is holding Amulet Coin.
Then you can just beat him up.
Because of the Amulet coin the boy will give you double money. He is really supposed to give you 7000$ but now he will give you 14000$
Now sell the nugget. So you will get over 20000$!
Hope this helps!