What is the cost of developing technical specifications and UI schemes for a mobile application?

Question

User interface

It is required to develop technical specifications and schemes for the user interface of a mobile application (Android, iOS) for a security agency. The interface scheme will be the same for 2 operating systems.
A similar application to understand the amount of work:
https://play.google.com/store/apps/details?id=com....
How much should I charge from a customer? Or is it stupid to calculate by hours of work (but I'm not very experienced in this matter, I can drive the price too much because of the time spent learning various aspects of the work)?

Answer 1

I don't know how long it will take me, maybe someone else knows?

No, no one else will be able to evaluate your knowledge, level, time and other price components either. Name the amount that suits you, which you think is fair.