Basically domain names are very cheap, it is about 10 usd but if you want a best domain you can pay more money to register. These domain names sold as noted in the news and media:
Hotels.com $11 million in 2001
Porn.com $9.5 million in 2007
VacationRentals.com $35 million in 2007
Fund.com £9.99 million in 2008
Internet.com $18 million in 2009
Insure.com $16 million in 2009
Insurance.com $35.6 million in 2010
PrivateJet.com $30.18 million in 2012
Sex.com for $14 million in 2014
360.com $17 million in 2015