If you're looking to make loads of PokeDollars in the quickest amount of time as possible, then you're in luck because there is an old trick for doing so in almost every Pokemon Gym. The secret lies within first defeating the Main Story and becoming the next Pokemon Champion. After doing so, you will be able to challenge the Elite Four as much as you would like. In addition to earning loads of Pokemon Experience by challenging the Elite Four over and over again, you will also earn tons of cash since they are the strongest trainers in the entire Kanto Region!
You can even earn double the amount of money in these battles if one of the Pokemon you use in them is holding the item known as the Amulet Coin. You can only obtain one Amulet Coin during your adventure through the Kanto Region, and it will take some effort in order to be received. This is because the only Amulet Coin to be obtained is from Professor Oak's Aide on Route 16, and the requirement for being gifted it is 40 Pokemon Species being registered to your Pokedex.