“You can roughly workout what someone is earning on their YouTube channel by looking at their total amount of views, dividing it by a thousand (being conservative), and then dividing that number by the amount of years they have had their YouTube channel online. You can see this in the “about” section of their YouTube channel.
Let’s take PewDiePie, for example. He is the most subscribed YouTube user of all time (again, at the time of writing this).
He has had over 10 billion views since April 2010! At the time of writing this, it is at 10,722,699,688!
Taking 10,722,699,688 and dividing by 1000 works out to just over ten and half million dollars! At the time of writing this it’s 2015, so dividing 10.5 million by 5 is around 2.1 million dollars. This means that up until now, PewDiePie has averaged over two million dollars a years from Google ads alone! Amazing! In reality, it probably started as quite a bit less and increased over the years. Only he really knows.” – Joe Dimon
Where did the thousand come from? See here.
P.S. See this article for a more detailed description on earnings on YouTube.