You know.  Ang is pretty darm smart.    Just look at Ennis's hats.    In the beginning of the film Ennis has a new crisp Cowboy hat on.    As the movie goes on, his hats become dirtier and more droopy.
I think this is not just to indicate his financial status, but his emotional status.   Not that he is dirty and droopy, but emotionally worn out and beat.