Real Madrid do not want to spend money on a goalkeeper at the moment as they may have to spend a lot of money next season. Real Madrid has a big chance of signing Mbappe next season, so Real Madrid must have enough money to complete the deal with Mbappe. In the absence of Thibaut Courtois, Real Madrid already have a number of goalkeepers, and with Real Madrid bringing in Kepa Arrizabalaga from Chelsea on a season-long loan, it doesn't make sense to spend extravagantly on a goalkeeper at the moment. If Kepa Arrizabalaga is not injured, he will be seen as the main goalkeeper for Real Madrid throughout the season.
Why should Real Madrid sign Mbappe next season and prepare money to take him to the Santiago Bernabeu? because if they want Real Madrid to sign him this season, they only hope to get the Frenchman for free because next season his contract runs out at PSG, there are indeed continued talks about his fate at PSG with a clause that he can leave next season, but everything can change including Mbappe changing his mind.
Real Madrid were supposed to buy Mbappe this season at a price set by PSG but it wasn't done and I think it's not just about the high transfer fee that Madrid are considering but also the huge salary they have to pay every season for this player. PSG have given too much value to Mbappe so it will be difficult for any clubs to make him a player at that club.
It is rare for a goalkeeper to be valued at a high price, usually only attackers and midfielders are expensive, but Real Madrid can get Kepa on loan is very profitable for them because Kepa has enough experience at Chelsea and the Champions League title he got a few seasons ago only he was not as the main goalkeeper of chelsea at that time because Mendy was still Tuchel's first choice at that time.