Facebook probably would charge users directly if it were acceptable to do so. Unfortunately (in my opinion) the optics around providing that choice would be even worse than not providing that choice at all. People would portray this as akin to racketeering, making users cough up the dough if they don't want to be tracked. Ironically, concern over Facebook's data collection inhibits their ability to explore direct monetization methods.

Also, the value of this information isn't secret: Google and Facebook are public companies and publish their revenue numbers, no? It's not hard to divide by the number of active users to get an average value per user.