Think "bottom-up/biological" (psychiatric) vs. "top-down/behavioral" (psychological). Many behavioral disorders have components of both, which means that you either need a psychiatrist doing both med management and behavioral work (e.g. therapy) OR a psychiatrist + psychologist working together to address the problem. The second option can be quite a bit cheaper, since the behavioral treatment is usually far more time-intensive, and psychologists' time is generally less expensive.