It's possible to eat out cheaply and healthy in SF but it requires years of practice (in my experience.) You'll eventually figure out where to go and what to get where based on the day of the week (specials), time of day (portion sizes) and time of the year (seasonal items.)
For instance, I just happened upon a place in Mission Bay that makes a custom salad (literally, they make it in front of you) with meat, cheese and two other items for $7.50. This salad must weigh 1lb. It's huge, healthy, a great deal and could be split into two meals.
Don't give up!