What cost sharing is, and what it is not
A pilot who is flying anyway may share the direct costs with whoever comes along. Anything more than that is something else, and it is not allowed.
Cost sharing means everyone on board, the pilot included, contributes to the direct costs of a flight that was going to happen anyway.
The pilot earns nothing. No money moves through the platform. And what a seat costs follows from the aircraft's hourly rate and the number of people on board, never from what somebody is willing to pay.
That is why Rightseat checks the rate before a flight becomes visible. Well above what comparable aircraft cost, and the pilot hears about it. So far above that the arithmetic no longer works, and the flight does not go online at all.