Yea I've been trying to track down HoD and CotM these past couple weeks. Average price on a used but 'complete' copy of HoD is about $35. Some people are trying to sell sealed copies for between $50 - $150. AoS seems a bit less. CotM however can be had for about $15 from what I'm finding.
Interesting. Its like I realize Castlevania games like Rondo and SotN have been very desirable but man, is the 2D Castlevania market heating up a bit even further? Perhaps since Konami's 2nd 3D PS2 offering, fans are getting sick of it all and restocking their 2D Castlevania fix or something.
EDIT: Found this gem of an auction. Some guy is selling an almost complete North American Castlevania collection here:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=8252055479&fromMakeTrack=true
I'm watching this one. Not gonna bid but be very curious to how high it will go.