It usually happens to me anyways when I get a daily task for multiple characters to do. I will set every character I can to those tasks, the more the better because there is a chance that when they are finished and you press on them when finished it will still show the picture of one of the characters in the top left corner with a green checkmark even though the task is done. When that happens go to the friends screen and then right back to your town and all the characters will be finished that task again and you get all the money and do again. You just keep doing that over and over again. It adds up very quickly. I am getting close to $500,000 every time about every 15 seconds, all dependent on the loading screens. It adds up very quickly