The opportunity cost of doing or getting something is best and fully defined as:

Check the explanation below

Explanation:

The tone of this passage is that of contentment. The author, despite not having everything and even sometimes sleeps in hunger, appreciates every good and bad thing nature provides him with. The words "fertile", "no difficulty", "free as the winds" contribute to this tone of contentment in the passage. If the words "travel" and "lonely as the solitary cloud, collecting and numbering plants" are used in the passage, this will change the tone of the passage to that of sorrow and discontentment. It would mean the writer no longer considers himself free but lonely and in isolation.