Let me tell you, there have been many years where I have pretty much survived at Comic-Con by eating at the Ralphs across the street from Horton Plaza. It’s almost a tradition for me at this point, even if I have the money to eat someplace better, at some point, I’m grabbing a corn dog out of the hot deli there. It’s a little bit of a walk from the con, but without a doubt it is the cheapest place to get a healthy meal within walking distance, corn dogs aside.
Still, if you’re looking to get some grub not marked with a price tag, Comic-Con has put together a handy guide in PDF format for you. It even has places serving convention specials.
Heck, while I’m at it, I made this video two years ago about cheap places to eat surrounding the con. It’s a little out of date now — there’s a Subway much closer in the Gaslamp District now — but it should give you some good ideas.