The main factor is not screen count
Portal cost is driven by business logic: who signs in, which data they see, what they can change and how the system responds.
What changes the scope most
More roles, integrations and exceptions require more discovery, backend engineering and testing.
- Roles and permissions
- Status transitions
- Data sources and APIs
- Notifications, payments and documents
- Security and audit requirements
How to get a useful estimate
Describe one core user journey, available systems and the expected outcome of the first version. That is enough to define boundaries and a next step.