A huge amount depends on how fussy they are about presentation and UI.
You *could* build most of that stuff in, for example, Drupal. It wouldn't be exactly plug-and-play, but you could get most of the functionality from contributed modules and just spend your time integrating.
The trouble comes when you realize that contributed module X works, but only sort of, and the interface is awful, and your client doesn't want the "email this article" link in at the bottom of the node but rather in a block, and the block needs to be styled, and -- AAAAAGH!
So if your client is not fussy about UI (and I think that's foolish) then maybe it's not too expensive. But if they are, then it depends on exactly what sort of modifications they'll want and how easy or hard it is for you to do them. That depends on the system, the contributed modules, and your personal skill.
Sorry this doesn't give you a specific number, but hopefully these are some good things for you to think about.