If I remember properly, making an app highly depends on the versatility of the app you're looking for. Will it be available in multiple operating systems (i.e. IOS, Android, etc) and what mediums will it be compatible with? The range is virtually too large to encompass unless you jot down the specifics.
Another good way of knowing how much is by consulting one of the main types of developers that actually make the: a .NET developer.
Being .NET developer is one of the coolest professions imo because it feels like you're working with a start-up every day. Making new apps, websites, and etc.
I'll link an article I read about them
here for anyone interested in knowing more!