€25 million euros seems like a fair price. No matter how good Donnarumma is, no club would pay the amount PSG is asking for a goalkeeper that has got just one year left on his contract. Also, PSG should know they don't have the upper hand. If they don't sell him this summer, they would still pay his wages and lose him for free next the summer. However, if they sell him right now, they would get some money back, which can be reinvested in the squad.
Given his performance last season, perhaps €25 million is still considered cheap compared to a player who hasn't proven himself as consistently as Donnarumma. The only reason PSG would sell Donnarumma at that price is because his contract expires next season, so letting him go in that situation wouldn't be profitable for PSG. They're also not experiencing financial difficulties and are probably not like other teams who only consider profit when selling players they deem no longer necessary for their managers. Honestly, this is not what we expected because Donnarumma was a player who contributed quite a lot to PSG last season, but every decision has consequences.