My favorite startup metaphor (originally applied to Groupon) is that the basic "get users, then improve margins" model would justify selling dollar bills for $0

My favorite startup metaphor (originally applied to Groupon) is that the basic "get users, then improve margins" model would justify selling dollar bills for $0.50. After all, there's tons of demand, and it's clearly a real business opportunity because the users all stick around as you move towards profitability! Then you hit that final price hike to $1.00, and discover just how nonlinear demand can be...