First, I know little to nothing so keep that in mind. I went through the same process a year ago. Actually started 2 years ago. I found much what B&B said. Boats, houses, two different things. The valuations from surveyors varied greatly. When looking at one surveyors low valuation I asked him if he could show me comparables he could not. Even as a newb, I was the one spending hour upon hour deep diving on every website and source looking at boats. I knew the list pricing ranges of the boats far better than he did. Yes, not actual sales prices but I used a 10% factor for a big picture view. Sold 10% less than listing. Sure there were/are outliers, great deals as well as highly overpriced boats but..big picture I found that I was pretty close. My point is that most likely you know better than he does as you have been the one searching and looking. Plus...what factors are important to you that you are willing to pay for? For me it was a "northern" boat that hadn't spent 30 years in the Florida sun, FL 135's, teak decks that were serviceable, and an engine room that showed signs of care and not just sprucing up for a sale. I was willing to pay more for those. I am happy with my price even though some would probably say it was a little high. Again, those that said it couldn't show me any actual real life boats to prove their point.
Second thing I did was I banked away 10% of the purchase price for 1st year maintenance and repairs. Even though I found the "perfect boat" with no known issues, impeccable shape for 30 years old, I spent that 10% and a little more. Things seem to pop up out of nowhere. My experience is that is the norm not the exception.