RickB
Headphoneus Supremus
If you look at the Amazon.com pricing history going back to 2008, I'd say it's pretty rare, even recently.
http://camelcamelcamel.com/Sennheiser-HD-650-Professional-Headphone/product/B00018MSNI?context=browse
I think it's possible that they could be clearing inventory to make way for a revised model. Just my opinion and speculation, I have no information to base that on.