For many meals I will mix my ground beef half & half with either ground turkey or ground pork, which are both much cheaper in my area. Even breakfast sausage or ground chicken can work if they are cheap enough. This works great for things like spaghetti, hamburger helper or pasta type dishes, casseroles, or meatloaf. You can also use other "fillers" instead such as oatmeal, grated veggies, bread crumbs, cooked beans/lentils, or rice. My personal experience is that these fillers work best in things like meatloaf, meatballs, or Salisbury steaks, where you're forming the meat into a patty or whatever, and fillers are expected.
For pasta sauce I use a lot of vegetables, let the sauce simmer & cook down, and add a smaller amount of meat just to season & give texture. It doesn't have to be a meat-heavy sauce to have a good chunky, thick texture. You can also do sauce with no meat, and make meatballs instead (cut down with bread crumbs, grated veggies, lentils, etc).
For tacos/burritos, I mix my pinto beans & meat together for serving. By mixing it in with the beans directly, you can use much less meat and still have a good filling.
I have a lot of 1940s & WWII era cookbooks, which are wonderful for "stretchy" recipes, because everyone was on rations! There are some really inventive ideas on how to feed an entire family on very little meat, and making the meat stretch.