When I was a student and had to carry snacks in my bag all day, I found that dates and almonds made a very portable, tasty, and relatively nutritious emergency food.

Salmon jerky sounds very good too, but as you say, it’s expensive. I buy dates and almonds in the bulk bins of my grocery store, and can get quite a large bag for not too much money. And it lasts forever, so if you don’t need it you can just leave it in the glove box or whatever.

Oh, also the dates are really concentrated (but surprisingly complex) carbs, which is sometimes what you really need when you’re running out of energy. Any sort of nut or dried fruit could be substituted, of course.