Received this expected value brainteaser today. No right answer but curious for everyone's thoughts.

Jimmy Buffett's picture
Rank: Chimp | 14

I have a box that either has zero or $100 in side. Each outcome has a 50% probability. It costs $20 to open it and find out. I only have $10 so I need someone to invest another $10 with me.

If there is nothing inside the box, we both get zero. In the event that there is $100 inside the box, how much of the $100 would you require as your return for you to want to invest with me?

Thoughts?