It really depends on the specification your friend asked for, as Rob says you would need to keep the data secure and that costs. As does having it fully featured, well-designed, and so on.
What did he want/ask for? These things would push the price up:
- Secure data and coding for the site
- Credit card payments system
- Full database features akin to facebook
- Any things he wants the users to be able to do, if they can edit their images than that would be expensive for example.
Scripts would ideally be ASP.Net/MS SQL or PHP/MySQL. Although there are others, those combinations are best