There is a simple formula you can use to calculate the amount of money you will earn writing for a website.
First go to Google and write site: and then write the site address. This will show you how many pages Google indexed about that site. The lower the number of indexed pages the better because that means there is not so much writing competition and each article you write will have less other articles competing for the same traffic.
Now go to compete dot com and search for that website and see how many unique visitors the site has. Now the higher the amount of unique visitors the better because that means your articles will have more readers and you will end up earning more per article you write.
Now to have real accurate data you are going to need to write 200 articles for the site and see how they perform. You need that many articles published because you need to have very accurate data and with just a few articles you could be lucky and get too many readers for them which in fact most articles may not have that many visits like that article has.
Finally you need to find the websites that pay you the most for your articles.
Be advised that most articles will see its traffic reduced over the time because as they get older, search engine ranks tend to drop as well as people's interest in reading it. The only exception is ever green content like facts and how to guides.