Although we often talk about food quality instead of food quantity/deals, I'd guess there are a few folks here who wouldn't mind knowing about a cheap lunch joint in SF that'll fill you up. Del Charro (formerly Manana) is right downtown, on the corner of Don Gaspar and Alameda (right across from the Supreme Court - can't remember the name of the hotel it's attached to, want to say Inn of the Governors). During the Legislative Session (approx. MLK Day to March 22), they're running an all-you-can-eat buffet lunch (including your drink) for $6.00, tax included.
They've had mexican food (chile, enchiladas, fajitas, rice, beans) when I've been there, along with salads, stews, and desserts. The salads I've seen have a pretty decent mix of fancy greens (though I've had one friend complain that this isn't always so) - that alone would run you $6 most places downtown. The food isn't anything to write home about (although I was impressed by the salad greens, as I said), but it's cheap and decent and, hey, all-you-can-eat for 6 bucks in downtown SF. Everybody I've directed to this deal has been surprised and happy about it. The staff is nice, so you'll want to leave a decent tip, even if it is a buffet.
So, take advantage of the swag intended for the legislators and their kind, and hit Del Charro for a cheap lunch. It gets a little crowded toward noon, so you might want to hit it about 15 minutes early. . . .