To use some real numbers, let's assume Bitcoin replaces US dollars.
Once they've all been mined there will be 21 million Bitcoins (and assume none have been lost)
There are approximately $10.5 trillion US dollars in the world (http://money.howstuffworks.com/how-much-money-is-in-the-worl...)
10,500,000,000,000 USD / 21,000,000 BTC = $500,000 per Bitcoin