Price for grassland behind the own property

Depends on whether it is an inner area and land expected to be developed or an outer area.

It is an inner area, but not land expected to be developed. To my knowledge, the area around the cemetery is not allowed to be built upon.
I am not even sure if it might not even be an enclosed property. There is indeed a path to the property along the cemetery, but that is probably a different parcel again.

At least land at the cemetery boundary is not unbuildable just for that reason

I have just evaluated various maps of the city. It is designated as agricultural land, as recreational garden area, or as other area (without development status). No idea if anyone could rezone it as building land and if it would then be considered anticipated building land.

The entire area was apparently still arable land 100 years ago, which already belonged to the ancestors of the current owners. In the area of our [B-Plan] there was a factory before the war, which was gradually expanded during and after the war by adding further parcels. After the reunification, it was closed and then demolished in 2012/2013, and a residential area was created from it. Along the access road to the factory, houses were gradually built from the 1960s onwards, so that only isolated parts of the once arable land remain.

Such plots are subject to very individual prices. If the owner does not want to sell, it is priceless. If it is a burden to him and he just wants to get rid of it, it is cheap. The question is also what it is worth to you. Simply go to the owner and casually tell him that you are interested in the property in the medium term. You do this repeatedly, also during chance encounters. No pressure, no hurry, no arguments for him - just sow a few seeds. As soon as the owner thinks about selling the property, he will think of you. Do not involve third parties, because if it gets around that you want the property, you can unnecessarily create competition, which also sows seeds. If you absolutely want the property, make an offer he cannot refuse. That is naturally much more expensive.

Addendum: If I really want something, I approach it with what hackers call "brute force." I might pay too much, but I get it. Money is a great lever for shaping things. Money only has real intrinsic value for those who have a deep joy in their account balance and its movement - and not in what they do with it. Lack of money, on the other hand, is a tyrant - only then does money really become important.

I think the above is a good plan. Let's see if he's ever around again, then we'll pass a beer over the fence and I'll drop the subject.
Of course, I wouldn't pay just any price. The property would only be a small extension for a few fruit trees and some vegetable growing and as security so that no one else makes intensive use of it. At the moment, we have privacy screens on the left and right of the terrace through garages and hedges and to the back it looks very open since hardly anyone is there.
This makes our property appear significantly larger than it actually is.