Some of my favorite foods that are healthy and filling---- avocado. I literally eat guacamole four times a week with no salt tortilla chips. I love quinoa as a super grain, brown rice, etc. Beans are cheap! We eat a LOT Of beans. I buy the low salt versions and really, we are all happy with beans in some capacity from a bag of dried beans turned into soup to a can of low salt black beans as a taco salad. I shop the specials when it comes to lean chicken, pork or fish.
So, a healthy diet really is doable on a low budget. It's the processed food that is often so pricey!
good luck and am happy to offer more suggestions if you let me know what kinds of things you are looking for.