The volume is such that no one person can "manipulate" the price. Even the Winklevoss twins cannot afford $10 million per day, every day, for last six months.
it only takes 200 coins to make the price spike or drop $75 difference..
not 200k, not 2million coins... just 200 coins.. which is $60k.
it might be worth you check order history and current live order values.. take btc-e.com.. LOADS of orders of 0.011btc..
the 3600 coin generation or the 13million market cap have no relevance on the crappy exchanges, estimates are that of the 3600 generated a day, only 600 end up being sold.. and of them 600 very little of that is sold on exchanges,