It depends. Generally, I find it more expensive, but then I'm one of the those evil corporate expense hounds who only sneaks into Arby's when he thinks no one is looking. Some of us wear ladies underwear, some of us snaffle low grade BBQ pork sandwiches. We all have our dark secrets. Ssssh. Mind you, I expense both.
Restaurants tend to be about the same price, but factor in the obligatory 20% you owe Heather-your-amphetamine-perked-server-today for interrupting you every time you thought you had got away with a mouthful of food, otherwise she'll stab you in the eyes with guns, as is the American Way. California likes its sales tax, and the Bay Area is a place with disposable incomes, so not the cheapest part of the US. Budget London prices, effectively.
Indeed taxis, which you may need, though getting around the Berkeley campus is not a challenge and there's transport into SF.
Don't be tempted by supermarkets, otherwise you'll end up sitting in your hotel room wondering quite what you are going to do with a family sized bag of potato chips, given that it seems to intended to feed the Waltons, and was oddly the smallest size they had.