I recently went to La Vaca, which is way down at the south end of the Ave, for lunch after classes. I waited in line, got to the register, then realized I didn't have enough money to get anything. I turned around, dejected, then turned around again when I remembered the loose change in my bag.
I counted my coins at the counter, but I still didn't have enough. The man who ran the place said, "It's ok. What do you want?"
I got my food, then he took my card (stamp it for free meals) and initialed in all of the remaining boxes. My next meal there is free.
On top of that, the guy who runs the HoneyBee Cafe (next door to La Vaca) is exceptionally sweet, enough that it surprised me the first time I went there and has surprised others when I've taken them there.
Finally, both of these places aren't quite restaurants-- you pay at the front, and a tip isn't expected. This makes the means end up around $6.