I built block divider walls between properties for many a year. Usually the first one to build built the wall and swallowed it. Sometimes the other property owner went in half.
But the ones that complained the most and caused the most trouble were the ones that contributed nothing and were already built and living there.
they got a free wall that cost a few thousand and they caused the most trouble I have ever seen in every way possible
There's an assumption that folks will be cooperative ~ alas, out in the horse country you have these billionaires who decide what kind of fence to build, so they build it and send you a bill for half ~ usually costing more than your land is worth.
One such situation went to court and the guy was forced to pay the billionaires price. Later on he and the billionaire had words and the billionaire shot and killed him. He walked.
The lesson is that when your neighbor comes to you with the idea of building a fence you should draw down on him then ~ not later!
Also, if the guy's building an expensive fence and hasn't talked to you about it, take whatever action you need to take then, not later. The courts will strip you bare.