Just steam some sushi rice, choose a filling, maybe salt the rice a bit if you want and form it into the shape with your hands. If you want wrap it with some nori, salt or put furikake on it. No need to pay the foreign Japanese food tax! It’s dead simple to make, and whoever is selling that ripoff onigiri to you is probably not even Japanese anyway.

(Admittedly the rice here does taste better in my opinion, but so it goes…)

In France you can find them in supermarkets for EUR 3.50 apiece.

They're also super easy to make yourself, just some rice, any kind of filling, and half a sheet of nori to wrap it.

They cost 1.99–2.69 € where I live (depending on the supermarket chain). A pair of soggy toast sandwiches is about the same price.

It’s not the cheapest snack in terms of calories per Euro (they are around 130 kcal), but still reasonable.