The examples are rubbed. The difficulty with such a large stamp is not the flatness of the stamp, but of the paper you’re stamping. I managed to get pretty good stamping results when the paper was resting on a milled machine table or granite precision reference surface (my favourite tool!), but these are not everyday objects, and if you want to stamp into a notebook I’m not sure they’d work at all. Rubbing seems to be the way to go.
Spitballing I’d say 20€ seems reasonable, need to check out how much the rubber sheet costs as I was using scraps from a free sample the makerspace had. I’ll make a separate thread so as to not clog up this one.