We are mostly vegetarian so my suggestions are maybe a little atypical. My big one is: make your own seitan in big batches using an instant pot or other pressure cooker. If you can find the vital wheat gluten flour used to make it in bulk, it is very cheap and high protein/low carb. We get the flour in bulk for $4/lb at the local health food store, so the cost per pound of the finished seitan is about $1.50/lb, which is cheaper than most meat we can get, definitely way cheaper than anything boneless/skinless (which I consider to be the most equivalent type of meat to compare to in terms of general nutrition profile). You can of course buy pre-made seitan but I don't think I've seen any brands/versions that I would consider frugal, and they're often quite salty or have a lot of random flavoring additives. My basic recipe is:
For one log seitan (4-6 servings, can typically fit 5 logs in a 6 qt. instant pot)-1 1/4 c wheat gluten flour
-1/2 c. pinto beans (optional if you are trying to go super low-carb)
-1/4 c. nutritional yeast
-2 c. water
-1 clove garlic
-1/4 tsp salt
-other seasonings of choice (I currently like 1/2 tsp paprika, 1 tsp each cumin and coriander)
1) Set up instant pot for steaming (1 c. water+steamer rack)
2) Mix 1/4 c. wheat gluten, 1 c. water, nutritional yeast, garlic, salt, and seasonings in a food processor. Add remaining gluten and water to form a wet dough. Roll dough into a thick log and wrap in parchment paper. Place log on steamer rack. Repeat for up to 5 logs total in instant pot.
3) Cook on high pressure for 20 min and then allow pressure to release naturally.
I also recently picked up a vegetable spiralizer for $10 on craigslist, and during the summer zucchini is so cheap, so we've been having zucchini noodles 1-2x/week. They are good raw, or sauteed a bit. So far this
peanut noodle salad is the family favorite (just use zucchini noodles instead of rice noodles). I've also done a version with sauteed tomatoes, garlic, and a little parmesan, and last night did a stir-fry with red onions, bell pepper, thai basil, mint, and cilantro (grow your own basil and mint to make this version frugal!).
We also eat a lot of eggs: omelets, frittatas, scrambles, or just poached on the side of a bowl of vegetable soup.