The price will be around 70 - 100 EUR. I'll figure out a competitive approach to multiple plates (eg. buy 2 get 3rd one free or similar).

The steel is 4mm thick, I don't think anyone sells seed backup plates that thick at the moment.
It is heavy but will not warp or deform after punching 24 words.

It's definitely something that you can bury in the ground and it'll still be there with readable words decades later.