I first went to La Vaca on the Ave (a location that has since closed). I have basically only ever ordered one thing from them - the vegetarian burrito with mole sauce. But it is absolutely delicious. Big, with tomatoes, salsa, lettuce, beans, cheese, and rice, and perfect mole sauce (not too sweet, not too bitter). It'll run you just under $4.50 for this burrito. There doesn't seem to be much cheap food downtown, so I definitely recommend this place for your wallet as well as for the big tasty portions.