React is a JavaScript library used for building user interfaces for web applications, enabling developers to create dynamic, fast, and responsive websites. On the other hand, React Native is a framework that allows developers to build mobile applications using JavaScript and React, but for iOS and Android platforms. While React focuses on web development, React Native focuses on creating mobile apps that offer a native experience.

React and Angular are both powerful frontend frameworks with different approaches. React is a library focused on component-based UI development with a gentler learning curve, while Angular is a full framework with more built-in features and opinions. The choice depends on your project requirements, team expertise, and specific use cases.

React can be used to build a wide variety of applications including single-page applications (SPAs), progressive web apps (PWAs), e-commerce sites, social media platforms, dashboard applications, content management systems, and complex enterprise applications. Its component-based architecture makes it suitable for both simple and large-scale applications.

React reduces development costs through component reusability, faster development cycles, strong community support, extensive ecosystem of libraries, better maintainability, and the ability to share code between web and mobile platforms (with React Native). This leads to reduced development time and lower long-term maintenance costs.

The cost of developing a React web application varies greatly depending on factors like app complexity, features required, development team location, timeline, and specific requirements. Simple applications might cost $10,000-$50,000, while complex enterprise applications can cost $100,000 or more. It's best to get quotes from development teams based on your specific needs.

Yes, existing web applications can be migrated to React. The migration approach depends on your current technology stack and application architecture. Migration can be done gradually by implementing React components piece by piece, or through a complete rewrite. A thorough analysis of your existing application is needed to determine the best migration strategy.