Sure we do. They're a public company; they routinely publish this information, as required by law.
https://investor.fb.com/investor-news/press-release-details/... says that in 2017 it cost ~$20 billion to run Facebook (see the "Total costs and expenses" line in the "Year Ended December 31, 2017" column). In 2016 it cost ~$15 billion.
According to https://zephoria.com/top-15-valuable-facebook-statistics/ (other places seem to have similar numbers) Facebook has around 2 billion users, which works out to $10/user/year for 2017.
Now many of its users are in poor countries and may not be able to afford $10/year. If we just look at the US, Canada, and Europe, we see ~300 million users in Europe (same source) and about 180 million in the US+Canada, for about 480 million users total. That's $40/user/year, or $3.5/month.
At $10/month, Facebook revenue from just their US+Canada users would more than cover their expenses for 2017.
Now Facebook _does_ have a 50% profit margin in 2017. So if they wanted to keep their revenue, not their expenses, they would need to charge double teh above numbers. That brings us to the lowest number you cite: $20/month if they only charge in the US and Canada. Of course I would expect the number of users to drop a lot of they did that...