Do you have a fridge? If so, you can just stick half an onion (not minced, obviously) straight in the fridge for a few days just fine. The exposed surface will dry out slightly, but that's not an issue if you're just using them for sauce. I usually use a whole onion per dish anyways, since I love onions, and they're a dirt cheap way to add flavor and some semblance of vegetables to a meal.

If you have a freezer, frozen peas are also really cheap and nutritious, and they basically only need to be thawed. Plain pasta with salt, little bit of butter or oil, splash of vinegar, some of the pasta water (all those starches in the pasta water will help a simple sauce come together), and frozen peas is really cheap and easy, yet somehow really tasty.

You can also bulk it up if you add beans. Beans are cheapest if you buy a bag of them dry at some place like walmart (I prefer pinto beans and chickpeas, but you can go with any beans you like) instead of going for canned. Just soak the dry beans in a bowl of water overnight on the counter, replace the water, then boil them on the stove for about an hour (until they're soft) and strain. Chickpeas also taste pretty good with pasta.

Other options if you have a fridge is to crack an egg into the pasta right after you've strained it and mixing it so it can cook in the residual heat. You can also chop up a hotdog and put it in your sauce. Surprisingly tasty.

You can get by without tupperware to store leftovers if you eat just one or two big meals a day; it allows you to do more in bulk with a one-pot meal, with less preparation, and you just eat it all in one go. I also find I can get away with leaving it on the counter for up to a couple hours and then finishing it if I don't want to eat it all in just one sitting.

Also, the cheat codes to making food taste good, even if not fancy, are salt, fat, and acid. Even plain pasta tastes really good if it's properly salted and given a generous drizzle of oil and vinegar. Bouillon cubes are also a good cheat code to making things taste better for cheap. Heck, you can even buy a cheap bottle of vinaigrette salad dressing (e.g., italian or greek dressing) and pour that on your pasta for a basic pasta "salad".