Cheap Meals & Del Mar don't seem to go together, but there are lots of options for families with kids or just taking a break from the higher priced restaurants.
Some of my favorites are not necessarily in Del Mar, but are in nearby Solana Beach, Cardiff or Encinitas.
Some of my favorite cheap meals are Mexican Food, especially fish tacos! My favorite for convenience, selection and quality is Rubio's in Solana Beach.
Rubio's Fresh Mexican
A bit further North in Cardiff, is Las Olas. I've been going there for over 20 years for fish tacos. Its more of a sit-down place, still casual and reasonable - and right across the highway from the beach, with some great views.
For quick, cheap Mexican food, go to Roberto's at the North end of Solana Beach. I remember going there after a good surf session and getting the carne asada torta for a few bucks...
For pizza, try Pizza Port on the highway in Solana Beach, just North of Lomas Santa Fe. I remember getting a "growler" (1/2 gallon bottle of beer) of some excellent microbrew to keep in the fridge for a few days, and the pizza is good.