Well it is hard to say. If mileage is low and it is in good shape cosmetically, with a hose down to clean off dust, you might be able to start at 2000. IF the engine is free. If it is not free, drop 1000 instantly and the engine could be badly frozen and require a complete rebuild
A lot depends on your area. See the 77/78 are not as sought after as the 69-76 because of the styling changes. Personally, I prefer the 78 style over the earlier look. The main issue comes in that a buyer MUST replace the tires. They will be so old and hard that the rubber would be VERY unsafe. The battery will be a goner and all rubber bits will have a good chance of being shot.
The one big bargaining chip you have is the exhaust. If the chrome is in good shape and there are no rust holes or rattles, you could command $500 easy on pipes alone.
The ride IS the adventure. The destination is just to get gas!