You can’t arrive at an accurate estimate without laying down your requirements properly. You have to determine what stage your application is at currently. Is it just an idea in your head or do you have the prototype ready? Do you want to develop a mobile application or a web application?
Then you have to determine whether you need to hire a medical app development company or go with freelance developers. Unless you have an entire team who can take care of everything else, it is always better to hire a company because developing an app requires more than just developers. Even within this, you have to decide how experienced you want the company or your developers to be and whether you want them to have any particular skills. You also have to determine whether you want frontend developers, backend, or both.
The most important factor that determines the cost of developing a health app is the number of features you need in your application and how complex you need them to be. Once you have the answers to all these questions any good health app development company can give you an accurate estimate of what the cost may be.