Since serverless environments work on a pay-as-you-use model, you are billed for the exact quantum of computing resources that you use. It does away with over-provisioning and having to manage idle servers stolidly, leading to an enormous reduction in infrastructural costs. For a MEAN stack application, this means low expenses on database operations, API requests, and front-end rendering.

Serverless architecture helps in speeding up the development process because it brings to the table ready-to-use managed services which can be utilized by developers. This reduces the requirement of infrastructure management, enabling the MEAN stack developers to concentrate on writing clean codes with effective functionality rather than wasting time taking care of server maintenance. Since serverless functions are highly modular, this creates good synergy with the MEAN stack, which is also built around modularity. As a result, this will enable fast cycles of development and deployment.

Moving to a serverless architecture will require breaking down monolithic code to microservices, dealing with state across active stateless functions, and optimizing for cold start times. Moreover, service integrations, for example, connecting with MongoDB, would also have to be thought through carefully. AWS SAM, Google Cloud Functions Framework, and Azure Functions Core Tools can provide templates and best practices to help this migration along.

Secure authentication and authorization are handled by serverless architectures through the use of managed identity services. Services like AWS Cognito, Firebase Authentication, or Azure Active Directory offer strong user authentication and role-based access management for MEAN stack applications. These services guarantee safe access to the assets and data in your application by integrating with your Express.js backend in a smooth manner.