Master Card has a series of cute commercials that list a series of accounting items and costs leading to a priceless product. Cell phones are often advertised as being free. In economics, it is said that nothing of value is either free or priceless, everything has a price. Take something from your experience, that is allegedly free or priceless and use the concepts of accounting costs, economic costs, explicit costs, implicit costs, opportunity costs and sunk costs to explain why it is neither priceless nor free.