I'm afraid I can't help save to say that most 'cost benefit' analyses here in the UK, do results in saved costs to the government, but almost invariably do not result in any consumer or customer benefits. The cost reductions results in organizational benefits. However, the whole process tends to be presented as being of benefit to customers and consumers, and the end result is usually disbelief and incredulity.

Dr. Zimmermann, 

I am not aware of a specific model, but I believe that one can be created. i have designed or assisted in the design of a few portals.  The underlying perspective or theory will be key here.  In the pubic sector, (and as you know I am sure) is that benefits are harder to measure in monetary measures.  Costs of creating the portal is not so tough.  Therefore, the purpose of the portal is critical here. If the purpose is to  improve efficiency and to move transactions on-line, then standard economic cost-benefit analysis will work, and work well.  However, if the purpose leans toward improving governance (things like citizen participation, citizen feedback, response to citizen questions and concerns), then that is more problematic.  You might be able to draw on models from U.S. federal and state projects, especially from the regulation citizen feedback and approval process. This process in the US is heavily Internet-mediated.  

Please keep in mind that my answer centers around a United States context, which may or may not apply to you.  Let me know if I can be of further help. 

I hope this helps.  

Good Luck.

Michael Pippin  

I guess you can borrow from e-commerce literature. still, do we need such model. The literature indicates that some countries impose a simple micro fee on some services. Still, if we refer to other researchers when talking about e-government it is not all about service. 

So if we look into other models, like e-consultations, e-democracy, it might not be cost effective... so it might need a different perspective