Asking vs Selling price
You''re asking if there is a scientific process to determine what a boat is worth? You may not get any definitive answers here, since used boat values are market driven. If a new boat interests you, of course brokers/dealers will not reveal their bottom lines publicly. When bartering for a car, a dealer will never tell you his cost and profit margin.
I just bought a 18 year old, 33 foot Finland-built boat for 75k that was listed for 89k. My broker shared inside info on two exact model boats of the same vintage that sold regionally within 6 months prior for 135k and 118k and listed for much more. My surveyor valued the boat above the listing price . . . so, did I get a good deal? I feel like I made out OK. Unless, I see a similar boat selling for less $ months from now. But then it may not be as well maintained and have fewer upgrades. I certainly won''t beat myself over it.
Work with your broker to research current and past offers and selling prices on the boats you are interested in, when serious about one, hire a good surveyor and give the owner your best price.