It would be very funny if PSG sold Mbappe at a cheap price, Mbappe's previous value was more than 200 million how could he be left at a cheap price now? besides that do you think with Mbappe's contract extension by PSG they will not benefit? don't be too naive in this case because even if Mbappe's goal is to leave but with the previous contract extension it indicates that PSG does not want to lose their money. Even if in the end PSG will also not be too long in the fry from the price issue but still he will be one of the most expensive transfers in the end.
Yes, I agree that if they extend the contract, PSG will earn a large amount of money, PSG must earn their money, but that money will not come from the Club Madrid. Maybe Chelsea or Saudi Arabia are also willing to spend much money to get Mbappé. But Mbappé flatly refused because the decision belonged to him.
You should remember that Mbappé will not easily tie himself to the PSG club anymore. Now, the decision will belong to him. The most expensive transfer or the cheapest sale still needs time before we can answer.
That is not a parable because in the end at this time PSG has extended Mbappe's contract even though it is only a formality so there is no "if" in this case.
As for the future of Mbappe we will not be able to be that sure to determine his fate because in the end he determines himself. For the Arab club it will not be possible even when the transfer opened last summer, the 7-fold offer from the Arab club was flatly rejected and it will not be possible for Mbappe to leave with his current condition where he will still get more things at a big European club. And although several clubs are currently linked with Mbappe such as Liverpool and Chelsea but in the end everyone's interest clearly sees almost the same conclusion between Madrid and Mbappe because if we follow this discussion from when the domestic league is off and the transfer starts then Madrid is the strongest candidate.