Why Is This Book Worth $500?

An OWF reader noticed my mention of Cook's Illustrated's How to Make Ice Cream. A CI fanatic, she searched for it online (it's out of print). What did she find? That the book is worth about $500. Check bookfinder if you don't believe me. (The stains on mine probably diminish the value, but I'll bet it's still worth a fetching farthing.)

This book is small: 96 pages and about a quarter of the square area of a regular cookbook. It was part of the "How To..." series the magazine did when they first experimented with repackaging the recipes from the magazine. I prefer these little How To books to the new mainstream glossy books they publish, but these are not revolutionary texts. Why the hefty price tag on this one?

My only guess is that, if someone wants the whole set, this is one of the books they can no longer get from the publisher. The books were printed in small runs, mostly available to subscribers and in very select bookstores. Any other theories?