Located in a more residential community El Arbol is a good find. You get a full meal - salad (choice of fruit or vegetable), a bowl of soup, a main dish and a large drink (I was given a refill without any extra charge) all for $68MEX. Good deal and very tasty.
Also if you go to that area on Saturday morning there is an organic market (Slow Food Market) on the corner of Avenida Reforma and one block north of Avenida Colón (calle 33D). AND west down the block is an organic cafe (Café Orgánico). BONUS.
Cons: Fairly remote from downtown core