Since Santa Ana is my hometown and I'm somewhat familiar with the area, I figured I would spearhead this thread.  Cost effective and delicious food should be emphasized here, there are no shortage of hip (but pricey) restaurants in DTSA if you want to have that kind of experience.  Furthermore, these locations should be within walking distance or not very far to drive to.  Please feel free to post any suggestions.  Here are some recommendations:

Subway:  On third and Broadway (walking distance).  Self-explanatory, cheap and decent.  $5-$7

Wursthaus:  On Fourth and French (walking distance).  Great polish sausages at fair prices, their fries orders can possibly feed two.  Imported German and Dutch beers.  Open late.  $7-$20 (fries w/beer)

El Toro Carniceria-Meat Shop:  On First and Bristol (short drive).  Lots of stuff but they're known for their carnitas, so a pound or two of carnitas with a pack of fresh tortillas could feed a small group.  $5-$15

Mother's Market & Kitchen:  On Main and Memory Lane (short drive).  Actually not terribly cheap but figured I should include a reputable place for vegetarians/vegans.

Bari Pasta & Pizza:  On First and and Lyon (short drive).  Pasta, pizza, and calzones at affordable prices, they deliver.  

Fish Taco Truck (don't remember name):  On Main and Pine (short drive/long walk), location varies but is usually there for lunch and the afternoon.  Delicious fish and shrimp tacos estilo Nayarit besides other seafood.  $2-$3 per taco (cash).

This is what I have so far off the top of my head obviously yelp is another good resource, but I thought this thread would be a decent start for folks looking to eat decent food without breaking the piggybank.  Like I said, please feel free to make suggestions I'd love to try something new in the area!  I'll continue to edit in more options soon...