[hornetblack]

Trier made €0 notes with Karl Marx images on them to celebrate his 200th birthday... Which they sold for €3 each.

https://www.thelocal.de/20180418/karl-marxs-birth-city-sells...

[huhtenberg]

It's the zero-euro commemorative banknotes. Purely collectible items. I have a couple from France and Italy, and they were €1 each.

https://eurocommemorative.com/en/13-0-banknote

[jansan]

We got several, too. What I like about them is the really good quality, including watermark and other anti-counterfeiting features. It's a nice souvenir, relatively cheap and doesn't take up much space in your luggage.