I love soft tacos, at home and at restaurants. Since a taco's filling is infinitely variable, the choice lies in your hands to make them cheap or expensive. You could buy fresh fish, for example, but that would up the price a bit. You could also consider a wide range of potential garnishes, everything from salsa to shredded cheese to cilantro to black beans. On the other hand, if you're looking to save money, the trick is to minimize. Use a filling that's hearty but cheap, and practice rigid discipline with respect to your garnishes.
'budget cooking' on Serious Eats
When I'm trying to pinch pennies, I often turn to random grains from the bulk bin of my grocery store. One cup goes a long way, and you're hard pressed to find any accoutrements that push a grain dish into the land of the lavish. But humble as grains may be, there's still something a little surprising about serving barley in place of rice, or bulgur instead of standard pasta.