ifishalot-
The boat hasn't been run in 20 years, so you can plan on going through a general service and carburetor cleaning on both motors, which will cost ~$600-$1000 in the Bay Area.
Be sure to look at the upolstery, canvas top, and any other fabric or soft parts for wear and deterioration, even though it hasn't been used for awhile.
The basic fiberglass hull should be structurally sound. Check out the hull fittings for tightness. The wood sponsons/spray rails at the sides of the rear will need refinishing at least. Check for severe cracks that would necessitate replacement.
Are the steering and motor controls intact and working? The gas tanks will have to be drained, flushed out, etc. Fuel hoses will need replacement, they get brittle and cracked with age. Don't try to run any of the old gas through the motor to start it! You will need a new battery.
You'll need to pack or replace the seals and wheel bearings, too. How about the tires? They are probably cracked on the sidewalls and in need of replacement. Is there a spare?
I wouldn't be surprised if the trailer needs touch-up painting over the galvanized finish here and there after 20 years or so. Does it have workable lights? How about hydraulic brakes? Does it have a tongue jack and does it work?
Are there mats or carpet on the floor? Any replacement necessary?
Whether or not the boat's worth $3900 depends largely on it's condition and the cost to fix it up. I would not be surprised if it cost something in the neighborhood of $2000 to restore it fully. From the looks of the manufacturer's standards, the hull would be well built like a C-Dory and good for a lifetime of fun. It would be a great boat for larger rivers like the Sacramento.
Joe.