My method is for really quick grinding.
Get your strongest pokemon with the best type coverage, such as Mega Rayquaza and give it an amulet coin, then use prize money o power level three on the elite four. If you want to be able to use the prize money o power level three constantly, just make sure your pedometer on your DS has registered 4000 steps and your opowers will refill 4x as fast as normal.
This yields 50-60k per battle if I remember correctly. Let yourself lose against Steven and get transported to the Pokemon Center right outside of the elite four, and repeat for maximum profit.
It's a really easy way to get a ton of cash if you need it. And if you're just using this for breeding money or something, then one run through the elite four will last you a while :3