The chart shows that there are significant differences in the price distribution per topic. First of all, there are many paid newsletters in the news category, most of them again in the $5 to $10 price range. Tech has a lot of free newsletters. And business and finance are topics where many authors charge more than $10. This confirms the trends in the examples from 2018.
So my suggestion would be to use both list size (current and potential) and topic (which determines list size) as guidance for monthly fees. Here are some specific examples from the most important segments discussed above:
1. Daily culture, list size 50K - 250K: $3-5
The Browser by Robert Cottrell
The Shatner Chatner by Daniel Lavery
2. In-depth news analysis, list size 100K - 400K: $8-12
Slow Boring by Matthew Yglesias
Popular Information by Judd Legum
3. Tech essays, list size 50K - 200K: $10-$15
Stratechery by Ben Thompson
Platformer by Casey Newton
Benedict’s Newsletter by Benedict Evans
4. Industry / financial advice, list size 10K - 50K: $15-$40
The Wolf Den Crypto Newsletter by Scott Melker
Marketing BS by Edward Nevraumont
Sidechain Society by STLNDRMS
There are probably a few other segments that will emerge over the next few months and years, but at least the above ones seem established in terms of format and price point.
Hope this helps to determine the monthly fee of your newsletter project. Feel free to reach out to me if you have questions at firstname.lastname@example.org.