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if I want to build app how much it will cost to me?

Depends on what you want build. what features you need in it.

app like which is control other mobail app like how much time other user spending time in other apps

you can already check that in OS settings

I know but I want to build some app like which is show other people's mobaile using time etc like that

what about privacy?

forget about privacy just now tell me about how much it will cost

totally depends on the derailed list of what you want to build. find a fresher if you want it nearly free. if you can make a business out of it, DM me.