The Martin depreciation scale for boats may be of help. Some type of initial point is needed, new price or replacement value. It's an average approach and does not account for quality of a boat's maintenance over time, upgrades, replacements, outfitting and other details which are specific to each boat. Can also be useful if there a several of the same model of different ages on the market. Just another tool useful for reality checks.
Visiting/inspecting a number of boats will provide a feel for the intangibles. How many to visit? Depends on experience with boats, systems and quality of information about the boats and patience.
If there is enough information another approach is to estimate a 5 year cost of ownership. We went through this exercise for several boats. Many boat specific variables including other costs to purchase (travel, surveys, taxes, documentation, post-purchase repairs, moving boat to home base), annual cost to use/maintain, depreciation, lost income and finally, cost to sell with expected sale price.
Attached is a typical comparison chart. Somewhat complicated but I can provide more information if there is sufficient interest.
Note that there are several other threads which address costs with various approaches ranging from detailed to 'can't put a price on boating experience'. Just depends on how you want to look at it.
First time posting attachments, not sure if it will work.