The people defending the guest here don't understand contract law. There's no mutual assent. The host didn't agree the box opener gets $700. He invited people to play with a box in his house, not to open it and win money. The guest assumed the host was running an illegal casino on residential property. He wasn't. Host owns the box. Guest gets his quarters back at best. If you think the host owes the guest $700, you couldn't pass the bar.