There are many variables that factor into what your boat is "worth", let me just throw a few out there for you.
The condition of the hull - is it structurally sound, what kind of shape is the finish in, has it ever been repaired and if so, what kind of work was done on it (quality or "patch job"). Is all the original hardware and windshield there, has anybody cut holes in the dash to install "accessories"?
Floor/Stringers/Transom - Many of the older boats have issues with wood rot, are these items still in good shape? Does the floor seem solid or give when you walk on it, are there stress cracks (or popped rivets in the aluminum models)in the 'glass indicating the stringers are shot and "giving", is the transom straight (most models), solid, and true?
Power/Controls/Electrical - What kind of shape is the motor in, compression even between cylinders, most recent maintenance including water pump impeller, any signs of overheating (discolored paint, etc.), lower unit (gearcase) damage or bearing wear, lube condition? Engine controls and steering operate freely without excessive play, wiring intact (condition of insulation and connections) and lights etc. operating properly?
Interior - What kind of shape are the seats and upholstery in, including the (many times) wooden structure underneath. What kind of shape is the carpet or floor in, mold or mildew present?
Trailer - General condition good, not excessively rusted or damaged, tires, wheel bearings, lights, bunks or rollers all in good shape?
Title/Registrations - Do you have the proper paperwork on the boat and trailer, has it been registered recently, will the new owner have a hard time trying to get it transferred and registered in their name?
Current market in your area - A boat that brings $1000 in one area may only bring $500 in another, and $1500 somewhere else. It's a good idea to check Glassifieds, Craig's list, local newspapers, boat trader mags, etc. to get a feel for where you can go with your price.
Two final thoughts - Pictures are worth a thousand words each (at least), and the real
bottom line is, it's worth whatever somebody is willing to give you for it.
Hope this helps.......I'm tired of typing and gonna go work on my boat now.