Basically <it> estimates with using following values.
estimated daily uniqueview(which is maybe the most important factor),
appraisal daily pageview,
estimated daily advertisement income(which will help to indicate the logical price of the domain),
domain google pagerank,
search engines google,yahoo and bing index status (Indexing is one of the hardest factor if you have a big website, so I believe that search engine visibility is very important calculating the website value).
domain age(which is very important ranking factor for search engines),
social media visibility, share count of your site on social media shows that your website has quality content(facebook, twitter, google+),
backlink count(google,yahoo,bing,alexa and total external backlink count. backlink quality is important than the backlink count but still backlink count is an important factor)