How can I approach writing an essay about how Gloria Naylor proves in Linden Hills that success comes at the expense of humanistic values?

You have put your finger on precisely what the two main characters (Willie and Willa) learn throughout the course of the book. I would begin your introduction with that idea and filter down to a thesis: Success can be had only at the expense of moral or humanistic values as evidenced by the minor characters of Linden Hills: Mr. Tilson, Winston Alcott, Reverend Hollis, and Evelyn Creton Nedeed.

Your body paragraphs would speak of each of these minor characters from Linden Hills respectively and always begin with a topic sentence. For example, the paragraph about Mr. Tilson should show proof that his need for money drove him to work numerous jobs and caused a fatal heart attack. Life is the thing that ceases to be valued in Mr. Tilson’s case. The paragraph about Winston Alcott should explain that he has married a lady he does not love simply to achieve respectability. Love is the thing that ceases to be valued in Mr. Alcott’s case. The paragraph about the minister, Reverend Hollis, should give examples of how he is a liar and a substance abuser. Truth is the thing that ceases to be valued in Mr. Hollis’ case. Finally, the resident of Linden Hills named Evelyn Creton Nedeed has a rich yet cold husband.  In order to cope, Evelyn uses chemicals to make herself look thin and to entice her husband. Eventually she uses chemicals to kill herself. Moderation is the thing that ceases to be valued in Evelyn’s case.

For your conclusion, reword your thesis and then present a new idea. Perhaps you could make a case that the main characters (Willie and Willa) are able to think clearly because they are not members of Linden Hills. They are outsiders who are able to determine right from wrong.