Its not that simple as it sounds. I have a feeling that Bitcoin value in the exchanges does not represent its value in the real economy.
I wrote a blog post about it: http://blog.leancrowds.com/en/2015/04/supply-and-demand-in-bitcoin-economy/
What do you think?
The exchanges can create bitcoin out of thin air if they are dominating the exchange like MTGOX. But now, each exchange can only control a small part of coins, if they create too much coins and crash the market price, the arbitrager will rush in and buy and withdraw, cause they to buy back coins from other exchanges to deal with the withdraw (Their coins out of thin air are not possible to withdraw)
But you are right, exchanges works exactly like banks, they have the possibility to create coin out of thin air. That's the reason people should stick with P2P exchanges thus get rid of the middle man. My favorite is localbitcoins, you have zero risk of market manipulation by localbitcoins
The exchanges can also create fiat money.
The P2P exchanges are exchanges which work in the real economy while the centralized exchanges work in their own economy.
My whole point was that the market value of Bitcoin against the Dollar is different between those two economies.