I'd go to one of those little hole-in-the-wall ethnic eateries. Some of them are every bit as good as the famous expensive restaurants, just without the decor and wine list. Chinatown of course comes to mind. Just walk up and down a few blocks and look at prices on the menus. Without alchohol, many meals can be had for under $10. North Beach has Italian food, some of the places just a little higher than Chinatown.

The pricey stuff is at and around the big hotels.