As
@dr morbius says, beans. We eat them daily, in desserts, sauces, main dishes, and salads.
We eat whole food plant based which doesn't include meat, fish, chicken, eggs, dairy, and oil while vegetarian might include some of those things. We also don't buy many, very few processed foods (soy sauce, worchestershire..). We save about 25% of our original food budget. We spend on more variety and volume of veggies and fruit than average shoppers.
If anyone doesn't eat meat, then start taking B12 but a similar % of meat eating people also suffer from B12 deficiency, so everyone might want to consider it anyways. 2000 micrograms once a week.
Different authorities say the savings for vegan and vegetarian is 16%-35%. If adding in faux milk, faux butter, faux mayonnaise, faux cheese, faux meat then that is as expensive as the original foods and there is no savings there.
edited to add: Imagine the money saved on health care when eating a healthy diet maintaining a healthy weight, avoiding chronic diseases.