Another good source is an insurance company. If you are looking to get insurance on the boat they will be more than willing to let you know what a reasonable value for the boat is.
They won''t be able to give you an exact figure as each boat is different depending on condition and equipment, but even having knowing the range will help. e.g. getting a boat in excellent shape for a mid-range price would probably be a good deal. Or if you find a fixer upper you will be looking at the lower end of the scale.