> It's fairly easy to establish what plans and kits are worth as the
> companies producing these publish the prices.

A published price has nothing to do with worth. It's simply the price they are willing to sell at. Most things are worth less then what they are sold at.

> I'm curious how one would put a price on a finished boat.

For what purpose? For insurance purposes, it's worth every penny I can get for it. If I can convince the insurance company that it's worth ten million dollars on a claim, great. Of course my boat is for sale. And if someone wants to offer me a price that tickles my fancy on a given day, the boat will become theirs. No set absolute value, if I've had a great day paddling, the price will be higher then if I've had a lousy day with it. And of course if I just want to sell the boat, I'll start absurdly high, and gradually lower the price until I start getting bites.