CarInsurance.com
LasVegas.com
Porn.com
Internet.com
 
Correct Answer: LasVegas.com

A brief explanation why…

According to information released in 2015 by domain name investor George Kirikos, back in 2005 Vegas.com paid $12 million in cash to acquire LasVegas.com and agreed to part with a total of $90 million for the domain over a lifetime of payments spanning 35 years from 2005 to 2040. Both Vegas.com and LasVegas.com host tourist information involving the Nevada city.

After the initial $12 million, Vegas.com paid $83,000 a month over the next 36 months ($2,988,000), $125,000/month for the following 60 months ($7,500,000), and then $208,000/month for a final 36 months ($7,488,000). In June 2016 Vegas.com had the option to stop using LasVegas.com or continue paying -- it continued.

CarInsurance.com and Insurance.com were Internet marketing firm QuinStreet in 2010 for $49.7 million and $35.6 million. Internet.com sold for $18 million (well behind the $30m sale of PrivateJet.com in fifth place), while Porn.com sold for $9.5 million in 2007.