Some did the math - the price Facebook would need to charge every user would be $11 a month. Sorry I can't remember WHERE I read it though.
[1]: https://investor.fb.com/investor-news/press-release-details/...
https://www.adexchanger.com/platforms/facebook-made-almost-2...
https://techcrunch.com/2018/04/15/would-it-make-us-love-or-h...