Does Starlink pay SpaceX for launches?
For SpaceX it’s critical to maintain a steady cadence. If they don’t, they lose institutional knowledge. If the cadence drops below a point, their effectiveness at reusing rockets will also drop, defects will creep in, and they’ll have higher fleet churn.
Yes. Amount is actually irrelevant, since if they underprice it SpaceX loses launch revenue, and if they overprice it Starlink looks less profitable.
The more important takeaways are that SpaceX's near-monopoly launch business is profitable but not nearly as big as Starlink, and Starlink is a good business but not one to justify a trillion dollar valuation