The process is simple, safe, and efficient. It can be performed relatively early in the game and is conveniently in the path of the story.

Let's get right into it!

For this method to work you will need the Magnet Stone, which is relatively easy to obtain.
The Astral Charm is not necessary, but may yield better results (I have yet to actually confirm this though).

If you already have the Magnet stone you can just skip the obain sections.

After defeating Lubella the Witch of Decay just head right as far as the path will take you. You know that you are heading in the right direction if you find a witch on a boat.

At the end of this path you should find a pier with a wolf on it, kill the wolf and pickup the item on the ground.

Now for the fun part, make your way towards Cortillard, that witch with the room full of cats.
As you make your way there, you will come across a room with a long breakable floor and enemies that drop from above.

Before you cross, equip your Magnet Stone (and Astral Charm if you want) and simply run across. Wait at the other end and enjoy your hard earned munnies. The next room over has a save bell in it, I recommend giving it a whack before heading back into the room to do the same thing in the opposite direction. Rinse and repeat.
Each run can give anywhere between 30-60 munnies.
Have fun!