Do the classic: Cream cheese and cucumber slices. Cucumber’s not that cheap, but you can make quite a lot of sandwiches with thin slices.
Edit Quark is also great. Just a bit of jam and a spoonful of Quark on a toasted bread is surprisingly refreshing.
Also not to put on a sandwich, but cabbage is pretty cheap. Also Potatoes. You can not only boil‘em mash’em & put’em in a stew but a lot more and they are pretty nutritious.
Also my lazy ass, who doesn’t want call every extended family member for medium important news: just tell ist mom without NDA and wait for a few days.