The cost of developing a mobile application can vary depending on several factors, such as functional requirements, design, app complexity, technologies used, scope of work, geographical location of developers, and other factors.

Mobile app development typically includes several stages:

1. Planning and requirements analysis: Defining the application's goals, functionality, target audience, and technical requirements.
2. Design and UI/UX: Developing the user interface (UI) design and optimizing the user experience (UX).
3. Development: Writing the application code, implementing functionality, testing, and debugging.
4. Testing: Checking the app for errors, compatibility with different devices and operating systems, and compliance with security requirements.
5. Release and support: Publishing the app in app stores, monitoring its performance, updates, and user support.
   

The cost of developing a mobile app is usually calculated based on developers' hourly rates and the overall scope of work. For an accurate price, it's recommended to consult mobile app development specialists and provide detailed project requirements.

Here are the main differences between developing separately for Android and iOS, as well as developing a native app:

Developing separately for Android and iOS:

Advantages:

• Each version of the app can be optimized for the specific platform's features.
• Ability to use tools and programming languages optimized for each platform (e.g., Kotlin/Java for Android and Swift/Objective-C for iOS).
• More flexible control over design and functionality for each platform.

Disadvantages:

• Increased development time and costs for maintaining two different codebases.
• Need to coordinate updates and bug fixes between Android and iOS versions.
  

Ordering a native application:

Advantages:

• Full optimization of the application for each platform for optimal performance and user experience.
• Access to platform-specific features and capabilities without limitations, which can be particularly important for apps requiring high performance or specialized device capabilities.
• Better integration with the operating system and platform ecosystem.

Disadvantages:

• Higher development and support costs due to the need to create and maintain separate codebases for each platform.
• Additional time may be required for development and testing due to the specific requirements of each platform.

The choice between developing separately for Android and iOS or ordering a native application depends on the specific needs of the project, budget, timelines, and the app's monetization strategy.