Not exactly 30€ but starting from 50 (when you subtract the 20 or 30% discount of some of the cheaper models):

https://www.lidl.de/de/matratzen/c11480

Edit: Here's the german discounter store archive, it lists past sales of the three largest chains Lidl, Aldi north/south. Just search for "Matratze" and you can check past sales up to 15yrs ago:

https://www.discounter-archiv.de/suche.php

Hard to say without trying them out. But by the looks of them, I would say those are more like sleeping pads than what I would consider a mattress.