NameBio.com is the price history data website. You could use this to ascertain your buying/selling prices. Point to note is that marketplaces take a cut from the seller which ranges from 10 to 20% for the Western market. It could be 3 to 8% for the Chinese market. These prices are the sold prices and does not represent the net the seller gets.
NameBio.com does not record sales below 100 USD.
In this video walk through we are trying to show you how to check the current market price of a Chinese premium LLLL.com and 5 number .coms with a few variations. This could help you master Namebio.