Okay, I found an estimate of world M3 at $75 trillion, which, if it's at all accurate, makes my numbers not horrifically far off.
ETA:
http://gizmodo.com/5995301/how-much-money-is-there-on-earthFor the narrowest definition, or what's called "M0", that includes only physical money, paper bills and metal coins that constitute currency. That figure is around 5 trillion dollars. The next step up is M1, which includes all the physical money, plus quickly accessed money like that in checking accounts, and comes in at $25 trillion. M2 includes M0 and M1, but also pulls in stuff like savings accounts and CDs under $100,000. That figure is around $60 trillion. And the last figure, the $75 trillion M3, is much more abstract and not often cited in official figures. It includes institutional money market funds, long term deposits, and other stuff rich people possess that can somehow be spent but confuses the rest of us.
ETA:
But actually, I think M1 or M2 is a better measure for this than M3.
Using M1: If M1 =~ $2.5e13, then, at 100%
BTC, 1
BTC =~ $1.2 million
Using M2: If M2 =~ $6e13, then, at 100%
BTC, 1
BTC =~ $2.85 million
The 21mio. bitcoins are the monetary base. It is "physical".
1. When I have 1Mio. btc and deposit it on a bank, M1 grew by 1mio. to 22mio., because I have 1mio. which I can withdraw all the time and the bank has 1mio. in theier physical power.
2. When I lend 1mio. btc to you and we make a contract that you pay me back 1mio. in one year, M1 stays the same, but M2 grew by 1mio.
So does it make sense to take the M0 number, the monetary base? Not really, because Bitcoin is not a money like we knew it. Maybe we need no banks in the future. Less lending, because no manipulating of the money supply. etc.
But this is all just hypothtical. I think it is possible to go to 100 000$ in terms of purchasing power of 1$ today. But that's it. And this number we reach only if everything works perfekt. Not only the state is a thread. Google etc. could come up with a better money. As so can do other cryptocurrencies.