There are more variables than you think
You never know all the variables. Maybe the husband died, and the wife wants to be rid of it. What about the pristine boat in "like new" condition, whose owner is going through a divorce, and does not want the spouse to have the asset, no matter what it costs him? Maybe the owner has already bought another boat, and can't afford the yard bills for two. After looking around and getting a feel for what the kind of boat you're looking for should cost, make an offer. If it's rejected out of hand, it's too low, and you've learned something. If there's a counter-offer, maybe you're in the ballpark.
If it's accepted, you may have been able to get the boat for even less. In any case, any offer should be made subject to a survey. The survey will turn up something which can be used to bring the price down further or which can cause the deal to fall apart, according to what YOU want. Each boat is different and each owner is also different. It's a totally free market. Take advantage of it if you can, and don't get hung up on formulae that don't work.