Comment by chao-
In the US, a rule of thumb for restaurant economics is that only about 25-35% of an item's price is the cost of ingredients, when you average over all menu items (of course some items better margins than others). The rest goes into labor, fixed costs, etc. It varies a bit by region and by market segment (e.g. fast food vs fast casual vs fine dining), but not by too much.
For McDonald's fries it's certainly much less than 25%. These are a high margin item, I wouldn't be surprised if ingredients costs is only 5% of that €2.99