This doesn't really count as extremely frugal, but you can stretch it to feed a lot of people. It is the recipe I tend to think of first when I am feeding a crowd. Well, it's often just us and another family or two, but often that's upwards of 20 people. So, I guess it's pretty frugal if it will feed that type of crowd.
Chalupa (this is my mother's recipe and version)
**You will need a slow cooker to make this recipe!
1 ~4lb pork roast (I've used all sizes, though. It really depends on what I find and how much I feel like paying)
1 lb dried pinto beans
2 cloves chopped garlic
1-4 oz can diced green chilies (these can be found in the Mexican food aisle of your grocery store)
2 TBSP chili powder
1 TBSP ground cumin
1 TBSP salt
1 tsp oregano
Mix all of the ingredients in your slow cooker, cover with water, and cook on low all day (~10 hours). You will need to check it every so often to be sure there is enough water... I often have to add some because the beans absorb the water as they cook. Before you eat, remove any fat that you can and shred the pork.
To serve (and I usually do this buffet style), set out the following toppings: tortilla chips, chopped tomatoes, shredded cheese (what you would choose to put on tacos), shredded lettuce, avocado (or guacamole), sour cream, salsa, diced onions, and black olives. We usually put the chips on the plate first, followed by the pork mixture and then the other toppings.
This also works well as a modified potluck. I have made the pork mixture and then asked guests to bring various toppings. Also, if you have some leftover, it does freeze well. You can also cook some more pinto beans and stretch it even more to make a second meal.