The Cost member represents the difficulty in traveling the node. Usually cost is influenced by or even equal to distance, but it doesn't necessarily have to be. Cost can measure a number of things including:
Difficulty in crossing due to terrain
Difficulty in crossing due to risk (enemies, thieves)
Resources required to perform a particular action (if node networks are used abstractly)
The cost property is used by the NodeNetwork to find the most efficient path from one node to the next.