I don't think the salary has anything to do with this Messi saga. I believe Messi wants to be assured of the project of the club before he agrees to remain there. He wants a better squad and better management. If they can promise that they will do that then I believe he will sign with PSG.
He is also accessing the same thing with Barcelona. He is waiting to see if it will be possible to join Barcelona and if they can guarantee that they have a good project and he believes in the project they're building. It's what players do when they have offers from various clubs. Some might just follow the money but most often what they do is go for the club they feel has the best project for them.
Messi is willing to go to Barcelona and receive very little compared to what he's earning at PSG, but it doesn't mean he would make the same sacrifice for PSG because PSG is not in a financial crisis like Barcelona and secondly, because he doesn't owe PSG any loyalty.
Right now, situation is not clear about Messi and for all better view about this we have to wait until end of this season with most chances he will not go to Barcelona because of their financial situation which is not good, and they have to release too many players which high salary which can create more problems for them so things looking not easy for the both parties with this all we have just PSG option for the Messi but before extended contract he needs to work on few things which will give better shape to this club right now coach and few players are creating problems which is never been ideal to win big trophies and Messi love to win trophies and now if PSG want to have Messi in their team for more time then surely they need to give better shape to their squad and also having few changes in their polices as well.
Despite a tempting offer from PSG whose amount is unknown, Messi's agents and father have yet to give in. Various bids have been coming in from other angles, including ALhilal, but Barcelona seems determined to get Messi at all costs.
Messi endorsement and revenue to clubs is really worth it otherwise, I don't know why clubs are willing to spend large sums of money to acquire him, just like him and Ronaldo are old but the interest to have them in their cycle is strong, there performance has been on decline and yet clubs don't look at it.
Messi to Barcelona is seems possible everyday, because Messi agent has not given any light to the psg offer. Personally I don't buy the ideal of re-signing Messi to Barcelona, it means they have to let some young stars go. Because of Fifa financial fair play. It will cost Barcelona a whole lot of money to bring Messi back to the club. But they will gain more. The club isn't interested in what he will win for them, but for the endorsement and sponsor his appearance will bring to the club. Messi and Ronaldo are for business now, sign them with big amount and gain more.