For lack of a better way to decide what to pay I assumed that they did the analysis and thought $5 would be an appropriate amount after factoring in the people who'd pay more, less, or nothing.

I think it's a good anchor price and if I was feeling generous it's a low enough value where I'd pay double. At a minimum I'd pay their asking price because after getting many hours of fun out of the game with friends it's definitely worth at least that.