Answer:
Free if you do it yourself, and not include any services. Services can be like databases or hosting and they cost money. So if you run your app locally it should be free, but some hosting platforms will host your app for free. And for the database you can always use something like redis that can store data in user's device therefore that will be free. So it is possible to create an app which is free but if you do want to build a fast app that requires good hosting, so maybe that will be 10 to 15 dollars a month.