It's only worth what someone will pay them. When I sold my 2005 2500HD gasser I looked all over for the same truck with the Duramax. Most that were in good shape were in the upper 20's to lower 30's.
KBB and NADA don't accurately represent what these trucks are going for lately.
For me personally I couldn't see dropping 32 on a 12 year old truck when I could drop 42 on a brand new one, and I wanted a 2006 HD very bad.