Buy dried legumes to get proper protein. Both cereals and legumes have protein, but not the full set of essential amino acids. However they complement each other in this regard, hence people living their entire life on rice, beans, and some fish.

I’d also advise looking into cheapest vegetables and making a salad out of them. Where I am, it’s potato, carrots, and beets — coincidentally the ingredients for a traditional salad. They can be cheaper than rice.