I have only been once, but food costs depend upon a lot of factors.
We were able to find some really good lunches for $3 - $5 per person, and I am talking about multi-course meals, not just a quick nothing. Casa Comida Toñita I think it was. El Morrito (sp?) was very reasonably priced for lunch also, and Jessy's too.
Dinners ranged from fairly expensive (for me) like Prima's which was $30 per person to more reasonable but even better food at Guido's that was probably $20 per person (give or take). Of course I don't buy a lot of drinks other than water, so you need to factor that in.
If you want to splurge at La Mission (any of the three) it may cost a few morebucks, but there are good values to be found. Of course some of my group had a craving for something "homey" so we ate one night at T G I Fridays just because I wanted a steak dammit!
One night we bought BBQ chickens for about $6 each at a place out on . . . Was it Salazar? The fifth street from Melgar. . . down the street from the Francisco grocery store. (Geeze, I don't remember enough details, I need to go back for a refresher!)
In my experience, and that of my friends, we found that the people working at the Dive Ops know some of the better value places because they can't afford to eat at the Tourist places everyday.
Just my thoughts
Wristshot