I estimate about 6 million Bitcoins of the 11 million are "alive". Given all the recent news about Bitcoin it's hard to see price going down below the $10 -> $20 range we saw previously.
How can you estimate it? Can you reveal the mathematical formula behind your estimation?
It's based on a few things and is subjective to my own experiences dealing with SolidCoin and analysis of the Bitcoin chain. So just take it as my opinion (whatever that is worth to you) as it is very difficult to pass on the knowledge I have without writing a book on it