KC, the value of anything is what someone will pay for it. All the other "statements" of value are simply opinions, subjective opinions in most cases.
The only semi-objective guide is to see what similar boats, in similar markets, have actually sold for. And that's not always easy. In theory the BUC Book lists actual sale prices and (agian in theory) condition as sold. You can get a valuation from BUC's web site for free, so you've got one more number to consider.
Online sales listings (like Yachtworld) are *asking* prices, not sale prices, so
AFAIK BUC is unique although I've heard any number of people say the BUC prices are higher than real market prices "should" be. Dunno, since they claim to be reporting actual sales.
Sellers and buyers and surveryors alike may all be delusional. You offer what the boat is worth to you, and if they don't want to take it, you either find another boat or up your offer. Apparently your surveyor thinks the boat isn't worth the asking price, and your best guide to the logic behind that would be to ask him.