How does Nick's statement"you're worth the whole rotten bunch put together" show a change in Nick from the beginning of the novel?

It is important to answer this question in the context of characterization.  Nick is a dynamic or developing character.  In his life journey, Nick gains insight into human nature; the human condition: what he learns is uncomplimentary.   In the first pages of the novel Nick describes himself and his values.  He writes of Gatsby, “…..[he] represented everything for which I have an unaffected scorn” and, yet, “…there was something gorgeous about him, some heightened sensitivity to the promises of life…” Although Nick disapproves of Gatsby’s behavior even at the end of the novel (Gatsby is, after all, a criminal), Nick recognizes that Gatsby has an “incorruptible” dream and Gatsby has some sense of integrity and courage.  As the novel unfolds and the friendship between Nick and Gatsby develops, Nick comes to understand Gatsby – what motivates Gatsby.  Nick sees the purity and nobility in Gatsby’s  dream, which makes Gatsby ‘great’ in comparison to the bored, privileged crowd like Tom, Daisy, and Jordan who belong to the "rotten crowd" because of their selfish, materialistic, and cruel behaviors.  Gatsby is "worth the whole damn bunch put together" because he gave himself over to his dream in totality. When the dream dissipates, there is nothing left for Gatsby to live for.