As a person currently selling my 2004 Cayenne S it was bit difficult to try to set the sales price. Mine is for sale for just over the $10K at $10,5K. When looking to set the price I realized there are a lot of deferred maintenance issues that show up once you start driving them that has driven the prices down.
I learned this the hard way in the 2005 Turbo I purchase to replace the 2004 S. The main reason I upgraded to the turbo is to keep the air suspension my S has and to add the cornering lights of the Bi-xenon headlight to aid in my failing night vision. Not many older S's had air suspension and even fewer air suspension and bi-xenons.
The Turbo I got looked good, drove well, and checked out good with Car Fax. I thought I was getting a great deal purchasing the 05 Turbo for just 10K. But, all the issues I've had to deal with in the first two months has increased the actual purchase price to the $17K to $18K range a lot of similar year Turbos are selling for.
In pricing my S also went thru all the pricing services like KBB, NADA, and Edmonds. They vary WIDELY! KBB was the lowest valuing an S in excellent condition at only $7500. NADA was next at $11,000 and Edmonds at $13,000. I did notice that NADA and Edmonds had more complete lists of the options available and the higher prices were because of those options. If I chose no options using either NADA or Edmonds their price matched KBB.
There are lots of issues on these older cars that can rack up the repair costs. Besides the plastic cooling lines running thru the cam valley (which should have been replaced by now) there are wear and tear items that are not obvious when purchasing. Like brake lines, brake booster, cardin shafter (drive shaft bearings) suspension, various sensors including O2 sensors, various oil gasket leaks, vacuum lines and even things like intake gasket leakes, electrical problems due to aging wires or improper LED blub relacement and various little interior bits that can be very expensive to fix.
If you are good, and a bit lucky you can find one that has been well maintained and will be a reliable car that will serve for many miles (yeas) without incurring too much more expense as long as the maintenance is kept up to date. That was what I did withe the S I am selling when I got it 3 years ago. Failed doing that when purchasing it's Turbo replacement. Looked and drove great, but a LOT of issues popped up.
If you decide to get a pepperwagon, good luck and I gotta say...my
2004 S with air suspension is a nice one for a good price!