If you're trying to live the mostly rice life, I strongly recommend buying some cheap pickles (I can get pickled mustard greens for .99 a bag and that bag should have about 7-12 servings worth) and chili crisp ($3.99 a jar for off brand, probably upwards of 20 servings) or anything else salty and flavorful (furikake is great for this, so is nanami togarashi).
Canned fish is a great suggestion for value, but because of the mercury content of many fish, you should probably look into buying the cheapest chicken you can find (.99/lbs. locally) and chopping it into mince with some green onions, soy sauce and Shoaxing wine to fry and top your rice.
Eggs are fairly cheap and help make a rice bowl great. You can also buy shelf stable tofu blocks faaaairly cheaply for some variety.