The bitcoin is around 1000USD today even with very little adoption. I am wondering what its value will be if it becomes as common as other currencies (say USD).
The best way to derive the possible value is by looking at market cap of bitcoin as a whole. There is currently about 1.2 trillion in USDs in physical currency alone. There are many ways to count how much USD there is though.
M1 represents all of the currency in the M0 money supply, plus all of the money held in checking accounts and other checkable accounts, as well as all of the money in travelers' checks. In June 2013, the M1 money supply for U.S. dollars equaled about $2.5 trillion [source: Federal Reserve]. M2 is the M1 supply, plus all of the money held in money market funds, savings accounts and CDs under $100,000. In June 2013, the M2 money supply was about $10.5 trillion [source: Federal Reserve]. M3 is M2 plus larger CDs. As of March 2006, the Fed stopped tracking the M3 money stock as an economic indicator because it felt it did not add any information on economic activity that was not already available from M2 [sources: Federal Reserve, Federal Reserve Bank of New York]. All told, anyone looking for all of the U.S. dollars in the world in July 2013 could expect to find approximately $10.5 trillion in existence, using the M2 money supply definition. If you just want to count actual notes and coins, there are about U.S. $1.2 trillion floating around the globe.
so for a complete currency, the "market cap" is around 1 to 12 trillion, which corresponds to a price of $47,619 - $571,000 per coin, using the 21 million bitcoin limit as the supply
I think at some point, probably about the point you describe (where BC is as "common" as US$, assuming "common" means "accepted in as many places), where the question ceases to be "how much is a BC worth (in US$)", and becomes "what is a US$ worth (in BC)".
A item (e.g. currency, chickens) is worth what someone will pay for it. In some other currency.