Alternative currencies as an idea? Valueless. Restoring wealth back to the people has limitless "worth".
In use currently? Well, if you factor out speculation I'd give a bullish value of around 1 USD/bitcoin. Very few transactions NEED to be in bitcoin currently that are not illicit and Silkroad by itself doesn't have the volume to push the currency into multi-multi-million market cap. Never mind billion-dollar marketcap.
However with all this hype and speculation that bitcoin will root out all evil bankers and offers the chance for speculators to have a portion of a "future-currency" it's easy to see bitcoin in low/middle double figures.
Because the market is based about 90% on speculation, bubbles will form and collapse and as a whole the price will endlessly struggle to find a consistent "value" until it is more widely adopted. I doubt this will happen anytime soon.
Of coarse these figures are out my ass and anyone giving an accurate assumption of the bitcoin price is purely supposition.
TLDR: Price wont find a true value until mass adoption and speculation is minuscule/not needed. And if/when that happens you're good looking at cool quadruple figures