As a graduate student, or otherwise on limited budget, you may have noticed that meat is pretty expensive. I've noticed that I have cut down on my meat-intake just based on this alone. (There is evidence that this is both good for your health and good for the environment. But don't get me wrong, I love my meat. I just love it less when I have to purchase it.)
White meats tend to be the cheapest-- chicken and pork. Because of swine flu or other fluctaions in food prices, pork seems to have been really cheap recently, so I've been eating a lot of pork.
Chicken is pretty reliable-- I like the ease of fresh or frozen chicken breasts. I can slice them or cook them whole, or butterfly them.
Beef is the most expensive-- except perhaps ground beef. One can only eat so much ground beef, but if you love beef then it's a good way to eat it on the cheap. Plenty of options there-- tacos, chili, meatballs...
Fish is best fresh; I've never had any luck with frozen fish. But fresh fish is tricky. So that's for another time.