There are a ton of stories about people holding out for more money from developers trying to buy up swathes of land. Some of them hold out for so much (including the effective infinity of "simply will not sell") that it actually ends up more practical to build around them. This is the extreme, but here's some pictures of the results of that sort of thing: https://www.google.com/search?q=eminent+domain+holdouts

I'll say again those are the extremes, but it's a common local- or regional-interest story to read about someone holding out for more money because they're the last parcel needed for some development or something. Usually it's an easy "little guy sticking it to The Man" story for the local news human interest segments.

Consider 2 outcomes. One is that the project moves forward with the owner getting x+y for their land. The other is that the project doesn't move forward and they are able to sell it for roughly x.

There is at least a sense in which turning down y is irrational. Of course it doesn't have to be irrational, but if they believe they would be better off with y and are simply trying to get more, there is some degree of unreasonableness involved.