I'll bet that it's probably like a driveway or something that will just encroach on that area.
I mean, would you even notice? I'd guess, off the top of my head, that 5k would be fair.
I had a relative once who had something like this happen. A neighbor of hers had accidentally encroached on her land while building a driveway. They offered to buy the land. Must have been like a few square feet.
she refused and in the process forced them to tear up the driveway.
Yeah, bad karma, over such a petty issue. Such is my family...