We were casting about for an inexpensive dinner last night and happened on this tiny restaurant on Hidalgo, just north of Lopez Mateos. (Ironically, it's Quesadillas Hechas a Mano sign has been my laptop background for a year, but we'd never eaten there.)
It was just what we were looking for. Tacos, 17p each, chunky guac and homemade chips, 50p, plus naranjada and papaya water, 30p. Friendly service and good food at good prices. (We are getting a bit of sticker shock at the price increases at many restaurants so it's nice to find another place with the old prices still intact.) The hours said they're open 7:30 a.m. - 11:00 p.m.
They don't seem to be on TA and a forum search didn't turn up any posts but I found a Facebook page (https://www.facebook.com/lacuevadelacherna/) and I know they've been around for a year or more. We are going to go back. Seems like a great spot for a meal near Playa Norte.