The price for a Bitcoin, for trading, derives from the interaction of supply and demand. It is also determined by confidence in the system. The demand is driven by a variety of factors which include rules in the software run by the majority of miners. As the supply of Bitcoins increases over time this can raise value. Many people find Bitcoin a flexible medium of exchange, with low transaction costs, resourceful micropayments, and very little regulation along with, the ease of pseudonymous accounts, a worldwide market appeal.
Speculation as to the future of Bitcoins as an investment, over the long-term as well as the short term will hold some effect. Bitcoin has potential to decrease when people lose access to the private keys which are needed to spend Bitcoins as well as when people find inconvenience of using Bitcoins due to peculiar software, difficulty acquiring Bitcoins, and a relative lack of people who have Bitcoins to spend.
There will be a finite number of Bitcoins released, and they are released at a predetermined rate, this helps Bitcoin retain value due to continued demand balanced with supply.