A well-stocked, well-stored pantry is the best insurance you'll ever have.
Imagine you lose your source of income. You have no money for food. What do you do? You go to the pantry.
Society collapses? You go to the pantry.
War happens and everything is rationed? You go to the pantry.
Natural disaster occurs and you can't leave your house? No worries, you have your pantry.
A yearly pantry will also:
Ensure you eat a well-balanced diet
Ensure your health, as much as nutrition can
Turn your monthly or even weekly costs into annual ones
Increase your budget
Give you more time to enjoy yourself
Simplify cooking and meal planning
What you need to think about it is what you usually eat and how to store those products. If you want to be able to stock up a yearly pantry with €100, you need to focus on paying for the staples and getting the rest for free, either through farms, farming, coupons or foraging.
The basics I'd choose are:
Protein - Legumes and seeds (usually cheaper than nuts) [You can easily get about 25 kg each of these. Mungbeans are cheap and high in protein, as are lentils. Soybeans have multiple purposes and are also moderately cheap, even organic ones.
Carbs - Oats, I'd say is the number one carb to invest in. Cheap, easy to get in bulk, versatile and can easily be turned into milk and flour. Rice and/or potatoes and/or flour made from wheat/buckwheat/corn are also good options. 5-30 kg rice is quite cheap, as is the same amount of potatoes. Personally, I'd go for oats, brown rice and potatoes.
Baking soda - A couple of kg of this will get you through baking, cleaning and spa-days for the whole year. [Cheap and multi-purpose.]
Vinegar - A dunk or two of vinegar will take care of your cooking and cleaning for the entire year. [A very inexpensive investment.]
Oil - If you buy bottles of oil, it will cost you, but if you buy big jars (coconut oil) or dunks of oil, from bulk stores, it is almost as cheap as buying one or two regular oil bottles.
These items can be acquired, in the amounts listed, for €100, based on the prices here in Sweden. If you have good contacts with people who grow some of these items, good contacts with farmers, you have coupons or are savvy when it comes to finding grocery store sales, you may be able to even go under €100. However, do keep in mind that the amounts listed are for a family, and may very well last your family more than a year, up to three years at most, depending on how much you eat and what else you have acquired for your pantry.
With these staples you got your carbs, your protein and your fats down. With the beans and legumes you can also make a more protein rich flour, or turn them into tofu, tempeh, milk or yoghurt. The rice can be ground to flour and depending on the kind, it could also be used to make rice milk.
For your yearly pantry, you do want more than these five staple items. You want spices, herbs, some sort of sweetener and vegetables of course. Those are the easy ones, the ones you can both grow yourself, trade to get or simply get for free. You can read more about how to get anything else, food wise, that you may want to store in your pantry here on this site.
You can vacuum pack your food and store it in big buckets with lids, pet bottles or use mylar bags.
Stay vegan and stay frugal!