Those books are out of print and didn't have that many copies printed to start with. It's a demand/availability issue. As long as the price holds up, you can recover most or all of what you pay for them.
It seems that the remaining stock of Grady's book, which was being sold through his room, is gone.
I was paying $14.95 dealer cost for a book. Been out of print and when the biggest supplier of the book sold out the price jumped a wee bit. Last I knew it was selling for $150 used. I gave a dozen copies away over the years!
I talked to a friend of a friend who had written a couple dozen how to books. Some sold better than others but most had a pretty steady market. He was getting old so he built a midsized warehouse behind his house and stocked it with his books. He said when he died that was what he was leaving for an inheritance instead of cash. Interesting concept, I think it worked. Some of his books were the best in the field.
Hu