Speaking as someone who does this for a living, average of three comps found on sites such as Yachtworld.com will give you a base. Use NADA to adjust for additions or conditions. Use NADA figures to calculate annual depreciation as well. This will give you a general value. If you want accurate, you need to find sold comps and try to obtain the actual selling price. It is some work, but if you start with comps, or NADA to find your general price range, you can do the hard research
on the boat you decide on.