They refer to it as 'private nameservers', and it should be free.
http://www.namecheap.com/support/knowledgebase/article.aspx/...
http://www.namecheap.com/support/knowledgebase/article.aspx/...
Name.com offers this service for free to domain registrations: http://www.name.com/faq/create-my-own-name-servers
EDIT: I stand corrected, please see: http://news.ycombinator.com/item?id=3869727