Not OP, but ag land can be under €10,000 an acre (well under in some places) which puts it in the "pricey but not unobtainable for the serious tree-planting enthusiast" category.
Putting a seed somewhere has a pretty low chance of producing a tree some years on. Conditions need to be good enough, and there's fierce competition in the plant world, as well as animals and lawn mowers that curb growth.
Which is why I asked, what OP actually did.
I know folks who have planted trees on public land and on land owned by corporations, which seems cool but also kinda stressful.