Yes, Grand Central Market is inexpensive and portions are generally very large (one pound, $2 taco anyone?) but more than a few cases of stomach unease have been reported after eating here (I work down the street), especially at the less busy places. Lots of the cheap produce is often on the verge of spoilage, too.
That said, most of the Mexican places here are superior to the adjacent Ye Olde Burrito House, next to La Cita on Hill, which is a must-avoid-at-all-costs joint.
This tip has the address "Grand Central Market, 4th & Hill"