Beans! I don't have much money and mostly vegetarian, and like to buy a pack of dried beans (kidney & black are my favourites - esp black!), soak and boil the whole bagful and then put in the freezer in smaller batches. Can be added to almost any dish, mashed to be used as a bread spread, and then my favourite - season with a bit of malt vinegar, salt and perhaps pepper to be had as a snack on the side of some fresh veggies.
I like my homecooked beans so much more than the ones that come in a can, the pre-cooked are not only expensive compared but taste a bit yucky to me, and have too much salt!
I also recommend homemade seitan. I like to make that too in bigger batches and then put into ziplock bags and into the freezer.