| Buying an apartment/condo in Berkeley would probably be the most expensive thing to do, but if you are planning to stay in Berkeley long-term and can afford it plus school, then perhaps it would be a good idea for you. I think you'll have to look into real estate costs in the Bay Area (it is still very high and still going up in many places, in contrast to other locations in the US), your longterm goals, and your financial situation. Even if you buy a 2-3 bedroom flat with the idea that you would rent out the extra bedrooms, it is unlikely that the rent you could ask for would be enough to cover a decent portion of the mortgage or could be cheap enough for other Cal students to afford. If you are willing to look outside of Berkeley, say, downtown Oakland or El Cerrito, you might be able to find a relatively affordable studio or 1 bedroom condo.
Renting an off-campus apartment by yourself could be a good economical choice if you luck out, but keep in mind you would be paying for food and utilities, too. Renting one with other people to share would probably be the most economical choice overall. Renting a (furnished?) room in someone's house/apartment as a sort of boarding room would be cheap too (but IMO the least attractive option - might allow less house access, might have to abide by stricter rules, I don't know...).
I'm most likely going to rent my own studio or share a 2-3 bedroom apartment with flatmates, unless I get a spot in the Berkeley Co-ops. | 
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