I think Bitcoin price is determined by supply and demand. Supply is well defined and well controlled. New coins only enter the system through mining. Mining generates new coins as an incentive for proof of work. This process generates new coins at a fixed rate. The number of bitcoins in circulation at any time is calculable.

The media can also influence the bitcoin price significantly. Media hype can easily lead to an increase in the price, while negative news can lead to a decline in the price. For example, news about bankruptcy or hacks on bitcoin related websites and services can cause panic and disruption among bitcoin users, leading to price dips. Negative news about government involvement and regulations as well as news on the use of bitcoins in illegal dealings such as drug transactions and money laundering tends to have the same impact on the bitcoin price.