Does my hotel business need a mobile app?
 

How much would it cost to develop an app?

It depends on many factors. What exactly app do you want, on what platform it will be, it functionality and many other. As I know Android developers use Java language. But there are many another variants. I think if you want to create good app you need to find professional developers. Based on my personal experience, I recommend Reinvently company(https://reinvently.com).