am visiting San Francisco/Monterey/Carmel during the second week of June and am looking for great food at reasonable prices. haven't been to a city yet where there wasn't great food from under $20/entree and I doubt this trip will be the first (with your help, of course). we are open to any type of cuisine/setting so Bay denizens show us what you've got.
You should check the California board for Monterey-Carmel. Remember Pacific Grove. some really good spots there.
There are a a couple of hundred good SF restaurants that charge under $20 for entrees so you might want to offer some more criteria to narrow down the selection.
Visitors often marvel at the abundance of SF restaurants with entrees entrees are under $20. At a typicalyl delicious, mid-price spot the entrees will range from $17-$23. A few of my personal favorites are Incanto, Aziza, NOPA, Bar Crudo, Out The Door and Farmer Brown.
A good starting point for your research is the Chronicle Top 100, which includes entree price range in the synopsis (though this can be misleading for small plates spots): http://sfgate.com/food/top100/2007/
Of course, not everyone agrees with this list, but it's a good resource to base searches and more specific queries on.