Most cities are pretty expensive to live in. As a DN, the main thing I care about is how complex the visa process is and what kinds of visas are available. Then, the public transportation situation. The rest can be subjective, and expenses etc can be lowered by not living directly in the city.
edit: The thing about these sorts of rankings are they're just subjective. I would say Seoul deserves a significantly higher rating, but I actually speak Korean and value what Seoul offers. If you don't speak Korean or care about public transportation etc, then Seoul is going to have a much lower ranking. Similarly, Germany is just less appealing to me because I speak little German. (I know many people speak English there, but life is just far easier when you speak the primary language.)