(there is around 2 to 3 millions BTC out of 13million total on public exchanges).
I very much doubt it's even close to your figure. Post MtGox, probably less than a million BTC, and I think even 500k are on the exchanges.A more important number is how much bitcoin is held by speculators who operate on the scale of days or weeks, rather than minutes. Those traders probably keep their coins and money off-exchange, but will move them in if the price changes significantly. Thus the walls at 800$ or 400$ may be a lot taller than what shows in the order books.