Why do we value only that which has a price?

[Jennifer]

Napoleon Hill says that "anything acquired without effort, and without cost is generally unappreciated, often discredited." and that "one of the strange things about human beings is that they value only that which has a price."  He says that libraries and free public schools do little to impress people because they are free.

Why do we value only that which has a price?

Do humans value something that is acquired with effort, but no cost, or vice versa?