There is no single answer to the question, "How much is mobile application design?" because the price depends on the scope of the work that will arise. Knowing the factors that determine the cost helps you both plan your budget and compare the offers you receive correctly.
The first factor is the number of screens and streams. A complex 40-screen application with a 6-screen MVP does not require the same effort. The second is complexity: a map with a list-detail-weighted application, an application with payment and real-time data is at different levels.
The third factor is the need for a design system. The component library set up for a product to scale takes time at first but accelerates everything in the long run. The fourth is prototype and user testing; the fifth is whether or not brand identity work is involved.
The sixth factor is platform coverage: is it only iOS, or does it also require iOS + Android adaptations? The seventh is the revision and delivery format a regular, documented delivery to the developer is the unseen but most valuable part of the project.
For a realistic budget, you can use the bidding tool on my site or describe your project in a few sentences and contact us; after clarifying the scope, we share a transparent price and calendar.