This is one of the biggest open-ended questions there are when it comes to SharePoint (the biggest being "What is SharePoint?").
In order to be able to produce a good business case, you need to be able to get under the skin of what the client wants, and also know very well what SharePoint is capable of doing out of the box AND what is and what is not possible by customisation.
The basic underlying main features (in my opinion) of SharePoint are:
Built in security and authentication mechanisms
Built in support of the Windows Workflow Foundation
Built in search capability
Alerts and notification
Item-level versioning and roll-back
Integration with line of business systems
Presentation of data
Scalability of hardware
Any business case for SharePoint is pretty dependant upon specific requirements of the client. As a solution, in order to have successful return on investment, it must "solve" a business issue, otherwise you have a solution looking for a problem.
Back to your question, for an "Internal Portal", or Intranet, or whatever term you like to use, SharePoint has, right out the box, authentication measures for Active Directory or even a SQL Database of users (using Forms Based Authentication). You can direct content on your portal to specific "groups" of users using Audiences. You can surface information stored in your Line of Business systems in SharePoint, and present this in your portal, or direct it to specific users using Audiences. The users of your SharePoint based portal can leave feedback and comment on content pages, as well as leaving tags, using their own expertise to classify content.
Everything in that last paragraph can be down using tools that come with SharePoint, using the features that are available in the front end UI, with no customisation or special code needed. This could be a good start to your research, as you can compare these features, typical of any portal, with the cost required to custom-build a solution which does this from scratch.