Costs Create Small World Architecture in Networks

Where do small-world architecture in networks come from? According to Jeff Clune, "Once you add a cost for network connections, modules immediately appear. Without a cost, modules never form." This issue of a cost would also explain why market economies are self-organizing scale-free networks with small-world architecture. And not just market economies, but all large-scale social networks. Social interactions all have costs, which means the most efficient network architectures will evolve. The existence of costs is most obvious in economics, but you will find costs in any network -- whether social, neural, or molecular.