To make an easy $40,000, head to Slot Ranch Casino in Sandbox mode and kill enough zombies to unlock the challenge on the stage. Before starting the challenge, go behind the curtain and find the flying saucer. Bring it with you and start the challenge. When the challenge begins, start the flying saucer. The zombies will be lured into 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.