The prop you will need for this farm is the Dojo Sword Rack, as everytime you break this item a Ninja Sword will drop. The easiest way to get it is from Wu's Dojo which is a free DLC build you can instantly unlock.

To Build the Farm first place down 5 - 10 Sword Racks and use the copy tool until you have placed 100 Sword Racks. The reason for this is that you are only able to hold 99 of an item at a time. Now save it as a Build. MAKE SURE THE BUILD HAS A FLOOR!!! If it does not the Flatten tool will not work as intended.

You can now Instantly place the Sword Farm down as a build and erase it by using the "Flatten" terrain tool. MAKE SURE DELETE OBJECTS IS ON OR THIS WILL NOT WORK!!!! You may need to move the "Flatten" terrain tool up by turning the "virtual floor" on and pressing the "space" key to move the tool up. This will cause the builds to disappear but will leave behind around 100 Ninja Swords. Now you can pick up all of the swords until you have 99!
Now to turn this to money you will need to wait til nightfall for a night time trader to come. If you have the Sky Spinner unlocked you are able to skip almost instantly to night time. You can then sell the swords to them for 40k studs each, or 3.9 Million studs for each Farm. Continue doing this and you'll be a Billionaire in no time!!!