Most top doctoral programs want to attract the best students, and provide stipend support throughout their course of study. This may be in the form of fellowship, in which you have no work obligations, or graduate research or teaching assistantships which have work responsibilities. However, many times the research assistantship is paid from a faculty advisor's grant to support a student's dissertation work.
To answer your first question, Molecular/Cell Bio PhD programs usually pay of stipend of >$30,000 per year plus tuition and fees. Check out the housing opportunities on each campus website. San Francisco is quite expensive to live in, but UCSF is introducing special programs to try to reduce the cost of housing.
To answer your second question, getting your own funding looks great on your CV or resume and gives you some more freedom than if someone else is paying the bill.
Here's what my campus website says about graduate funding and housing:
http://www.grad.uci.edu/funding/index.html
http://www.grad.uci.edu/admissions/housing/index.html