--I am doing a business website where users generate and share
content.  The intent is for the content to provide information to
users that contributors feel is valuable, similar to how del.icio.us
users learn about websites other users thought were interesting enough
to bookmark, or contributors to Wikipedia share information on topics
they know about.

I have never done web development before, so I need some advice on
making estimates.  Assuming I use an open source content management
system as a platform, about how much time should I allot for from
start until launch, and what will be the major cost drivers of the
development process?