One of the cheapest on Castro is "Food Street" -- Chinese. Not as cheap is the ramen place, Ryowa Ramen. I like the Manila Grill further up towards El Camino (where there is usually parking), but then I lived in the Philippines for 3 years long ago. The challenge on the main part of Castro is parking can be annoying at lunchtime, although you could possibly take the light rail for 3 stops from Ellis/Middlefield -- one light rail ticket is good for 2 hours, so it will do your round trip.