Old LEGO sets can be quite pricey. Looking at the pieces most of them are still available brand new directly from LEGO. It's the few discontinued ones that usually hike up those prices.
Take the Forestmen camouflaged outpost (6066-1) for example. I was doing an inventory for it so I could build it next from my old collection. The set sells for over 200 euros in secondary markets. I was thrilled as I had only few pieces missing... until I found out I'm missing the two most expensive pieces of the set. The black and blue plume feathers cost almost one third of the total set price. Other missing pieces were few cents. Those feathers are the smallest pieces in the set, so easy to get lost. It's also almost the only set where those pieces were made for.
Other third of the set comes mostly from the minifigs and their accessories. The bulk of the set, the pieces that are mostly still available today, or included in many sets in the past costs almost nothing per piece.
Of course there is also some additional value in those old pieces that are still being made today. All that wear and scratches give them the real old set look. An odd new piece in the set really stands out and feels out of place.