ICv2 estimated 519 for Inferno Cross-Overs in Aug 2010 and another 349 in May 2011 (presumably a warehouse clearance sale).  
  
ICv2 estimated 495 for Inferno in June 2009.  I didn't turn up an additional month for the book on the Diamond charts... but I only did a cursory search, and there might have been a month somewhere that cleared a big stack out of the Diamond warehouse.