Henry, I enjoyed your book.
What I did was take its value when I bought the plane and then subtracted $10.00 for every hour that I'd flown it since then. I've only had my C150 for five years.
Earlier this year, I stopped at Fulton, Missouri for gas, and it was the first day that I'd finally reconciled myself to selling my plane. Another man was there who had just bought a Cessna 150. I mentioned to the FBO that I wanted to sell my plane, but not to the other man because I figured he would not be interested.
But before I left, he came out and wanted to learn more about my plane. Turns out, he buys small planes and flies them to Brazil to sell. He said he flies the island route. He talked as if the market there was stronger than in the US for Cessna 150s.
The price really does depend upon finding a person who is interested. I expect the market during winter, and especially during this economy, is going to be tough for selling a plane. No one is likely to buy a plane merely because it is a good deal.