For years, we've been giving away free autographed bookplates that readers can stick in their copies of our books. (We've taken a break from this practice recently but will resume when we publish our next book.) I would estimate that we've mailed out between 20,000 and 30,000 bookplates -- all of them really signed by the two of us and all absolutely free, including postage. Why did we do this? It just seemed like a good idea, and a nice way to thank people for reading.
But what suckers we are! As a reader named TL recently let us know, there's good money to be made on eBay selling signed Freakonomics bookplates.