Get to know a yacht broker. They represent the majority of cruisng boats on the market. You can look at listings yourself on yachtworld. Once you find the boat that you want to make an offer on, ask him to run the broker only version of Yachtworlld called Soldboats.
Soldboats gives the reported actual sales prices for boats sold through Yachtworld. I say reported because some brokers fudge their reported prices, but only if it is a new boat line that they sell.
As the market has been very volatile, only look at prices reported over the last year. Make adjustments for age, condition and equipmet to give you guidance as to what price