Well, it could be this book, Tilt, The Pinball Book: Home Maintenance, History, Hot Tips is ultra rare, but I'm guessing this is bots who set prices getting into a war with each other and slowly moving the price up over time until it is just stupid. The cheapest used copy is $450.
http://www.amazon.com/gp/offer-listing/0916870146/ref=dp_olp_all_mbc
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For more on this phenomenon see: http://www.michaeleisen.org/blog/?p=358