Play Money is used in several board games, one of these games is Monopoly® the most popular game on the market.
A standard Monopoly® game has $15,140. By September 2008, approximately 250 million copies of the game have been sold. This leads to $3.785 trillion Monopoly® bucks in circulation. The amount of money in a single game actually increased to $20,580, and since more copies of Monopoly® have been sold since 2008 I am going to round the total number to $3.8 trillion.
Comparing that value to the amount of paper currency in the world in circulation, it is actually very close. The world has approximately $4 trillion in circulation.