If there's a farmers market near you, try going to that. The one near where I live only sells organic food, but it's still cheaper than the grocery store. Also, they don't usually sell much processed food, and that's what's really expensive. As others mentioned, try to stock up on the basics like oil, butter, rice, flour, sugar, etc and then buy your protein and vegetables as you need them. That way you won't find yourself just eating something like a frozen pizza as often. Snacks and prepackaged foods may seem inexpensive, but considering how little you get in each packet, they're actually expensive usually.
A lot of times I just saute some peppers and add them to a can of beans. Then I add sour cream, cheese, cilantro, tomatos, avocado, etc to taste/ depending on what I have. In the same vain, I've made plenty of meals out of rice, beans, and sofrito.