Use the following trick to earn $40,000 each time it is done. Go to Slot Ranch Casino in Sandbox mode. Kill enough zombies to unlock the challenge on the stage. Before you start the challenge, go behind the curtain and locate the flying saucer. Bring it with you and start the challenge. When the challenge starts, start the flying saucer. The zombies will be lured in to the saucer. When 25 zombies get close enough and remain there, you will win $40,000. After the challenge, pick up the saucer and repeat the process as many times as desired.