So pretty much you want cheap, grab-n-go, and nutritious food? Unfortunately though that's just not going to happen. Finding foods that meet all three of these criteria is damn near impossible, if you want to have a quality diet you are going to have to work for it both in and out of the kitchen. What sort of appliances/cookware do you have access to? It's sad but the unfortunate reality is that most cheap convenience foods are absolute garbage for you and that most nutritious convenience food is priced exorbitantly. However with investing some time in the kitchen you can certainly make nutritious food affordable. Here's pretty much what I survived on week to week spending $60-70, or ~$100 ($50ea) when splitting food/food costs with my one roommate (which I highly recommend).
Protein:
Raw Chicken Breasts (always cheaper than frozen)
High fat % beef (Drain/Rinse->Profit)
Eggs (Scrambled eggs on errrythang)
*Look for sales on special cuts of beef/pork roasts
Carbs:
Potatoes (10lb bags are never more than like $5, sweet potatoes when cheap)
Oats
Rice
Greek Yogurt
Fats:
Butter
Coconut Oil (Best of these three options for sautéing due to high smoke point)
Olive Oil
Other/Micronutrients:
Spinnach
Broccoli
Garlic
Onion
Peppers (Poblanos, Bell, Jalapeno's)
I could make a hundred different meals out of this shopping list with the right spice cabinet and few extra ingredients like sauces, etc.