The value of bitcoin is going to depend on demand and usage. If the size and usage of bitcoins approached the value of major currencies, the value of a single bitcoin would have to be astronomical given the constrained supply.

There's currently $1.2 trillion USD in circulation as cash money, $2.5 trillion when you count checking accounts, $10.5 trillion when you add in money market funds, savings accounts and CDs. If the usage of Bitcoins approached the size and value of the dollar economy, for the 20 million bitcoins to replace the dollar in current usage, the value of a single bitcoin would have to be over $500,000, and over $1,000,000 USD if it became the single global currency. Obviously that's rather ambitious. But if bitcoins are ever to become semi-mainstream, the sheer demands of even a million new users trying to conduct transactions in bitcoins would easily drive the price to $10,000+.