SMS wrote:

My sister-in-law had a neighbor that wanted to replace their shared
fence and she offered to pay half, and he said to forget it, he'd put
the fence in himself. He proceeded to put in the world's most
expensive fence, then came back and asked her for half the cost. She
would only pay half of what a normal fence would cost. Besides being
expensive, he left too much of a gap at the bottom so small dogs
could get underneath.

Ah, one of the exceptions I discussed earlier. Ordinarily, when the neighbor
starts putting up an expensive fence - say one made out of marble blocks
topped with gargoyles, you SIL is obligated to voice her objection else she
would assent by silence to the grandiose plan.

Here, since the neighbor said he'd foot the bill for the whole thing,
including battlements and watchtowers, she should be completely off the
hook. In the worst case, she'd be liable for half the cost of a fence
normally found in the neighborhood. Plus court costs, lawyer fees, and
various notary charges.