Earlier quoted context omitted.
Depends on the audience size and makeup. I've seen tweets like this: https://twitter.com/petecodes/status/1548983470805876737 which talk about $400/month for a newsletter ad. Cooperpress has a lot of dev subscribers and I believe charges thousands of dollars for an add: https://cooperpress.com/advertise/ (run by https://news.ycombinator.com/user?id=petercooper ).
Thanks. Are there good equivalent ad networks like AdSense for newsletters (so you don't have to establish a sales department?)
My experience has been "no".
I don't know of any networks that do newsletter ad placements; our placements have all been direct with the publisher. Some publishers have more than one newsletter so you can get some scale with them, but it's still direct discussion with the publisher.