First, if you can, try eating the potatoes with the skins. Less food waste, less time peeling, and it's where most of the nutrients and fiber are.
Next, consider something like canned beans. Not "baked beans" because those are mostly sauce and honestly kinda gross. Like a can of black beans or chickpeas or red beans or black-eyed peas. You can season them however you'd like, but they're good plain too. Versatile and can pair with almost anything. High in protein and fiber. And they're affordable.
Another option is dried beans which are the cheaper option per bean but they're more work to prepare because they need to soak. Lentils don't need to soak though, and a pound of dried lentils isn't too pricey.
If you can afford whole grain bread, it really does have much more nutrients than white bread. If you have a high metabolism, your body is just going to burn off all the empty carbs from white bread. Butter can add some easy fats too.
Oatmeal is another affordable option. Not the packets. Get the giant cylinder of instant oats. It'll last you many meals, and all it takes is some hot water. You can adjust to your preferred consistency, whether you like it thick and clumpy or a bit more soupy is up to you.
If you can eat nuts, find one you like and get a big bag of them. Larger quantities are usually cheaper by weight. Walnuts are pretty affordable. Lots of omega-6 fatty acids. You can even crush them up and put them in your oatmeal if you like that.
If you're not opposed to poultry, you can get a carton of chicken broth and drink maybe half a pint of it at a time. Heat it up first of course. It's very gentle on the stomach and can stimulate an appetite. Also provides lots of good fats which are what your body needs if you want to put on weight and keep it on.
If you don't eat meat, then vegetable or mushroom broth could do for stimulating appetite, just be aware that the fat content will be lower.
That's mostly what I can think of. If you have troubles with digestion you might want to consider live foods/probiotics like yogurt, miso, stuff like that. A little bit pricier but it can make a big difference if your GI tract just doesn't feel right. And again the larger containers are usually cheaper by volume.