There is no "Blue Book" of value for older outboard
motors...what a particular motor is worth on a specific day is where a seller and a buyer cease negotiations and either shake hands or walk away.
I have seen a number of good restorable outboards
at my local metal recycler (some of which I bought)
Someday demand for old outboards may catch up with
supply...that day is not yet except for race motors
and pre WW1 outboards.
Most old outboards need servicing, repair or parts
replacement...something few people are able or
interested in doing. imho