Extranet Costs, Benefits

Any ideas, experiences regarding Extranet costs, benchmarking.

Extranet should cover following:
Plan, realize and roll-out a product information platform offered to customers worldwide as a standard service.
Datavolume: 20GB
Clients: ca.500

Any ideas regarding development, operation costs. What is the financial benefit, what subjects has to be determined?
Who has experience with Extranet Project Management?

Thanks for any comments, suggestions