It depends on two factors: supply and demand.

How much of each there is is a result of the psychologies of thousands of different individuals, which each react differently to news about bitcoins, changing situations in their own finances, the price movements of bitcoins caused by other individuals, etc. The price is therefore essentially not predictable, nor can one even predict how much of a change a particular large news event will have on the price. This is analogous to pricing of stocks, futures, forex markets, etc, except that in those markets rather than dealing with the psychologies of thousands of individuals you are dealing with the psychologies of millions, which allows statistical analysis techniques to be much more useful.