I think that both NADA and BUC Value can be helpful guides but neither one can give a definitive value on any particular boat given the multitude of factors which may influence price.
In addition to the above references, I would suggest that you also refer to Yacht World and Boats.Com and try to get a range of prices of boats similar to the one you are considering. See if the boat you are looking at falls within that range. Also, remember that local where the boat is being sold may likely have a big effect on price. Finally, keep in mind that extras such as electronics may be valuable additions but might not effect the price of the vessel proportionately.