Question: Who decides the price of Bitcoin? and why Bitcoin prices are different in each exchange?
Bitcoin market price is changing with the balance of Supply and demand.
The value itself is created because users believe Bitcoin currency is useful to the society and there are places which accept Bitcoin as payment.
As Bitcoin is mined at a decreasing rate, demand must follow this level of inflation to keep the price stable.
The market price is very volatile comparing to other currencies. (February 2018) It is because of the small scale of the market, and relatively a small transactions and news can affect the price directly.
Different Bitcoin prices in each exchange
There are different Bitcoin market prices offered between brokers and exchanges.
It is because the price is decided by the exchange according to the participants of their market. They may adjust the price as they want and also to keep the balance of Bitcoin market.
Often the price differences can reach over $100.