Demand for certain parts of that setup are much higher than others. The one year only 60 Exhaust manifolds (as Dan pointed out) bring a premium and get scooped up fast, the air cleaners, carbs, linkage and small parts are also easy to sell. The most common parts are the long tubes as they are the parts least likely to get tossed over the years and more of them are around than the rest of that stuff. It can be a challenge to find a buyer for an entire setup because many people already have some or most of it already and are trying to complete theirs. All that said, 4-5K is not out of reason for a (Very) complete setup, however finding a buyer can be difficult.