What is the most profitable career that involves studying the brain.

If by profitable you are mentioning financial returns I might suggest getting a Doctorate. You can study the brain and help people with a Masters Degree (which is what I have done) but the financial return (generally speaking) is less than if you have your Doctorate. If your primary interest is helping people and your secondary interest is the brain, you can get a PsyD, which is a doctorate in applied psychology. If your primary interest is the brain, and your secondary interest is helping people, you can get a PhD, which is a research focused doctorate (that also allows you to do psychotherapy). If you're really, really, really interested in the brain you might want to look into Neuropsychology.