I would look into ResearchKit and finding an academic partner like UCSF or Scripps to work with.

Independent doctors would probably charge hourly or project based.

So the firs thing I wonder about this is not what the price will be, but how will you get mental health providers to integrate the app into their workflow? What is the ROI specifically that would make a company want to buy this? How can you justify the cost with the value provided? Is this something that can be billled for? What is the entire interaction between the patient and the doctor? Is the doctor going to get paid for using this? What is the benefit to the provider for implementing this? Once you understand the perspectives and experiences of every individual who is touching the product then you can start thinking about price.

The first thing you need to ask yourself is why you do not have good knowledge of the industry and how do you get this? Once you have learned about the industry you will find that there are hundreds of glucose meters and heart rate monitor companies. What would make your device different? The next question to ask is what is a niche area that you can dominate? What sensors are currently available for wearables and what is a new way to apply them to generate value? Most importantly you need to ask yourself what problem you want to solve and why. Use the Feynman method: Find the problem first, think really hard, and then come up with a solution.