Cost of mobile app development
This is the type of question that is bound to have no meaningful simple answer. It is like the question "What is the cost of building a home?". One can build a habitable shelter home for less than $1000, and can spend over $100 million to build a giant mansion. One can millions to build a very sophisticated mobile app.
It may be meaningful to talk about the low end of the cost. To make any serious app, a minimum investment of 50 hours is normally required. If the rate is $100/hour for a competent software engineer, this means the low end of mobile apps is $5K. Making a mobile app is much more than just writing lines of code.