Through some accidental research, I have found that it is possible to farm money using a custom level created in the level editor.

There are some caveats to go along with this for speedrunning, namely that there is nothing stopping you from playing levels created on another save file with access to more valuable worlds (since the world your level template is from determines how much money it gives you for the par bonus). Therefore, I have added a rule that if you use a custom level, it must have been created in the save file you are using for your current run. This rule technically applies to all categories, but the trick itself is only useful for Any% Unrestricted (and in Any% No Repeat levels, this wouldn't be allowed because in order to preserve the money you've earned from the custom level, you must beat the original level again).

Here is a tutorial for how this trick is used in a run: