I was going to say Wizards , but they don't have any more . Did you try Amazon . Lucky I got mine when I did last year .
The book has been out of print for nearly a decade, and being such a specialist's book, not many copies sold to begin with - just enough for the specialists, who were few and far between during this era mind you, to have their tangible copy of this convenient variety reference. But since Bust half dollar variety collecting has become more popular since the book came out, there aren't enough copies to go around. The supply cannot meet public demand. So, with this said, you probably aren't going to find a copy of the 3rd or 4th edition for under $50. But, occasionally, they pop up on eBay in BINs for well below trending price (this is how I got my copy), so that'd probably be your best bet. Just keep an eye out.
Never use the PCGS price guide for RAW coins very simply put it will always be too high. That guide is for PCGS graded problem free coins and is normally at full retail for them. A better price guide would be NumisMedia and only if you are able to detect problems and grade the coins close yourself. Link to NumisMedia guide.....http://numismedia.com/fmv/fmv.shtml