They haven't been that cheap since the 90s. 10 years ago, they were around $10.
You do realise that the article has actual photos of the menus from different years, right?
What's your hypothesis here? They photoshopped the images?
But yeah, it's likely an outlier in 2014 to be that cheap. More like $7-8.
Locally (not in SV), one can still find "cheap" burritos at Mexican joints. By cheap I mean $11-12.
In one local restaurant, they bumped up burritos from about $11 to $18 within a year. I stopped going there.
They haven't been that cheap since the 90s. 10 years ago, they were around $10.