This is a Google blog post from March 2018 with some interesting statistics for 2017:

[blog.google...]

We paid $12.6 billion to publishing partners in our ad network last year.

Google's own AdSense logout page states:

A few reasons why almost 2 million people have chosen AdSense.

Let's be simplistic, 12.6 billion divided by 2 million = 6.300 or $525 per month.

Obviously some people earn way more than $525 per month, like I used to, and obviously loads earn less than $525 per month, like I do now:-(

Nevertheless it does give a ball park figure that many have asked in the past however do bear in mind that this was for 2017.