It's not about salary because Mbappe rejected all offers from PSG so far according to news on media. He wants to find a new challenge and wants to be a biggest star in a team he is playing for.
With Neymar, Messi in PSG, Mbappe will not have that position. In Real Madrid, he will be the first brick of the Galaticos 3.0 in Bernabeu. I guess it is what Mbappe wants at this time and PSG won't be able to pay him too high salary. After they signed with Messi and pay too high salary for Messi.
Do we know what the offers were? I mean we know that there were few offers from PSG and he declined them all but we do not know exact numbers as far as I know, and I am 100% sure that it wasn't 50 million euro per year neither, nor was it 2x of what Real could offer neither, PSG could do that, they do not have la liga financial cap type of deals and they could offer Mbappe way more than what Real could offer.
Make it a stupid offer, make it so stupid that people will think that it would have been better to let him go for free instead of paying him that much, make it that big and then you get Mbappe for sure. I know 100 million a year sounds silly, but I just want to give an example; would mbappe say no to 100 million a year? Of course not, which means there IS a number that he would accept, maybe not 100 because that is too much but there is definitely a number out there.