My favorite portable protein takes some prepwork before you go, but it's cheap, light, and satisfying: Freeze a block of tofu (or 2, or 3) overnight, let it thaw, then squeeze all the water out. Slice into strips, and marinate it for a few hours in your favorite sauce (barbecue sauce, salad dressing, or make your own). Dehydrate it if you have a dehydrator, or you can bake it on the lowest setting with the oven door slightly open, and flip the pieces over evey 30 mins or so until the tofu is dry and crispy. Carry it in ziplock bags on your trip, it will keep for days, is full of protein, and weighs practically nothing.

If you can get an empty wide-mouthed plastic jar, fill it 2/4 with oats, dried fruit, granola, or anything else you want. Top it off with any kind of liquid (even plain water is good) close the lid, and let it soak for a few hours while you ride or sleep. Open the jar, and you've got a cheap, mushy, good-tasting snack. I also did this with some really good meusli that I found in grocery stores in Europe. When you finish, you just rinse out the jar, which weighs almost nothing when it's empty.

When I was in Italy I bought olives everywhere, and ate them by the kilo. They're cheap, delicious, and give you a lot of energy.