CT (this or Tacos? You choose!),
Either go for the dark meat, or whole chicken. Learning to butcher a chicken is a great life skill, and also produces several meals, out of an oven-roasted or braised chicken. A lot of times those meats are priced drastically lower. My wife isn't a fan of quarters because they're too gamey and even legs can be gamey, but she never has a problem with thigh meat. Consider some simple marinades to elevate the flavor or fight the dark meat flavor, if you're not a fan. We made meals for 3 days on 5 dollars, with chicken thigh meat, today.
Look for the cheap cuts, and slow cook them. You can make some great fajitas, stews, or BBQ beef sandwiches with some of the cheapest cuts of beef. The cheaper meats are usually tough, but some marination and a good slow cook will do them a world of good. Some of my best stews and soups are made with cheap off-cuts.
The more processed the fish is, the more expensive it is. A lot of times local markets will sell skin-on or even whole fish, at a lower price per pound. If you've got a nice light fish like a trout or something, you can (and should) totally cook it skin on. If you don't want to pick bones out, or take scales off, then you're going to want to look at the seasonality of the various types of fish and start shopping after the first big hauls come in - it's supply and demand. If you're not against freezing some fish, they make for great stews later.
As always, beans, rice,pasta and vegetables help make things last and can bring a lot to a dish.
In a popular lunch dish, a butternut pasta with smoke sausage, we use a 20/80 rule, and it's never once been an issue - 20% protein, 80% veg and pasta. learning to think of meat as an accent rather than the full coarse helps save money, without sacrificing flavors and variety.
There's some general principles.