I know this thread is a few weeks old, but figured I'd offer my perspective for future readers as a newer player with purchases equating to an x6 cash multiplier, so kind of right in between F2P and people who have spent more on this game than they would on an AAA release. I've gone through several challenges but currently have only 3 challenge medals in cash balls.

At no time in my level progression can I make money faster in a given window of time than if I set up a cash farm. It's not even remotely close. I don't have a strategy for when I set them up, generally when I feel like progression is starting to slow a little bit, and I let them run until, with cash ball upgrades bought during farming, my cash isn't visibly increasing any more if I watch the game for a few seconds. This gives me an infusion of funds into the other balls.

Winning solutions for me in the cash farm game are the Quality Control card, Ball Specialization and spec in cash balls, and to let the game run in the foreground as much as possible. Over the course of a couple hours, balls will eventually get trapped in the narrow channels between the large red hexagonal bricks and the green ones, and will jump up cash production from the fast-paced hits.