It's possible that the price of bitcoins is based on a demand only. What I mean is that people may want bitcoins for the potential and are demanding it as an investment which makes the price go up. But if people want it for that price, then isn't that what it is worth?
That's it, in a way. The demand is far greater than supply, but we have to agree that there is a supply. Out of the coins in circulation much is forever gone, lost on some unused wallers or damaged hard drives. Much is in the hands of early holders, who took their profits and have enough to live comfortably, so they have no need to sell their stashed coins. This limits the real supply, the one available to new consumers, to maybe 50% of the coins mined to date. The prices are just an illustration of the current state of affairs.