if you are using a broker, ask him for a sold boats report that he can get from Yachtworld, that will show, somewhat, what similar boats have sold for. Even with this system, there is some fudge, I know I sold a boat and then saw the report....it was NOT what the boat sold for by me....
that being said, you will need assess somehow what the boat is worth to YOU....a surveyor can help with a "value" but it may not be what you want to pay.
To quote an old horse trader, "he who names a price, first - looses..." You can make a lowball offer, and hope it doesn't shut down the seller, or you can negotiate a fair price, subject to survey, sea trial, insurance, lessons, etc....and then based on that adjust your offer or walk away...you will loose survey and trial $$$ perhaps to the tune of several hundred $$$.
If you are not using a broker, simply ask the seller what is the least he will sell the boat for today...but make sure you have your ducks in a row and are knowledgeable.
all the best
dave