That's $19.66* in the US (instead of $25.00!) and £12.83* in the UK (instead of £15.95).
So you know, I have no control over Amazon's discounting (and it doesn't affect my royalties). It has nothing to do with inventory clearance: the book is printed on demand, so there's always copies available.
So if you haven't bought the book yet, well, go get it! Over 110 drawings of ridiculousness!
* Prices updated as of August 1, 2013. They fluctuate daily. Fine print is fun!