As the value of goods rises, transaction costs rise, making the potential gains to a price mechanism large enough to provide an incentive to build one. The potential gains are the size of the transaction costs saved.
Like I've said before, the Net is going to resemble a bunch of interconnected markets much more than a bunch of interconnected networks. This is a great example of such dynamics.
Case study provided by Weinberger. - highly recommended if the above doesn't make any sense.