I don't like this question. It's akin to asking a startup, "What is your revenue model, and how will you be cash-flow positive?" Eventually every startup will need to answer that question, but asking it too early is more likely to stunt important and creative ideas that don't come with cookie-cutter business models than it is to reveal a winner. Much better to ask open-ended questions that get at the same thing: What's the potential impact of your idea? Why are you going to succeed where others failed? And if the answer is "We're smart and we have these advantages and everyone seems to like what we do" then it may be OK not to have the cost-effectiveness worked out in detail yet.