Opportunity cost.
Whose lives are potentially assigned lower prices when a drunk decides to drive home without waiting to sober up? (Pedestrians? People in other cars? The drunk? The drunk's family?)
Simple Quantity Theory of Money
What are the assumptions and predictions of the Simple Quantity Theory of Money? Does the Simple Quantity Theory of Money predict well?
Barter
There would be very few comedians in a barter economy. Do you agree or disagree with this statement?