The hiring cost of any developer, whether you want to hire an android or ios app developer. It will always be influenced by certain things including location, experience, project complexity and engagement model - full-time or hourly basis.
It very difficult to estimate the exact cost of hiring iPhone App Developers. But you can expect charges of developers for hours as below freshers developer would char around $15+, where developer with few years of experience would charge $35+ and more than 4+years experience developers will charge $50+.