An Australian friend of mine told me that in that country, pennies are thrown on stage to insult a bad live act. (In other countries, people used to throw rotten fruit.)
He told me the story of a relative of his who was on stage doing standup, when someone threw a penny on the stage. He walked to the front of the stage, knelt down, and beckoned to his wife who was sitting in the front row. She leant over, and he whispered something into her ear. He then stood up and said, "My wife's just told me that if I keep this up, we'll have collected enough money to pay our rent by the end of the evening."