The $15 Theory

The theory that charging your friends $15 to hangout with you is moral and socially acceptable.

Often the theory is used when you irresponsibly buy something unreasonably expensive, then find an excuse to make your friends pay for it through indirectly using it.

"Hey guys, I bought this new console, so every time you guys come hangout and use it, I'll need you to pay $15. It's only fair!"
"I know you're spending the night, so have you ever heard of the $15 theory? Yeah, I'll need you to pay me $15 to sleep on my new couch."