FWIW, I ran into a similar problem with the NGK OEM plugs in my VW. The dealer was charging $25 a piece (only 4 though). But I found them at Advance Auto Parts (special order) for $10. They have the HCH listed OEM plug NGK IFR5G-11K for $9.98. Not sure how wide spread AAP are.
http://www.partsamerica.com/ProductDetail.aspx?MfrCode=NGK&MfrPartNumber=3107& PartType=960&PTSet=A
Also, my car is a 2000 and by 2005 the dealer price finally came down to $10. Hopefully, the same will occur for the HCH, since these aren't commonly stocked parts.