Ideally, an app UI/UX cost anywhere between $50 to $500 per hour, depending on how complex the design is, what platform the app belongs to and whether it requires prototyping efforts. It also depends on the experience of the designer or the company, geographical location, as well as the number of hours required to design the app.
For an app requiring 100 hours of UI/UX efforts, the small design agencies usually charge between $50,000 to $100,000, while apps designed by established design agencies can cost anywhere between $200,000 to $500,000.