If you ask me about a number, I'd say 10-20% around the 75M they paid for him. And, yes, Barcelona sources have debunked any speculation. I tend to believe things are not as simple and rosy, especially in Barcelona midfield. A lot of people not happy with their squad status and a potential Messi the midfielder years to come. On the other hand - all of them are on a lucrative deals, Barcelona needs a shiny new 100M forward toy, praying its more successful than the last 3.
Add to this mess the internal struggles which encompass everyone from player through management and shareholder. Did I say that Barcelona is broke? The city todo.
Also unrealistic as Mbappe - come on, now. Players like Messi, Mbappe are a mission statement from the club. We got CR7 and I can guarantee you that if we offer half his salary De Jong's agent will call a terrorist organization to get him here if need be.
As for how much money we've got - it depends on what the plans are in our owners. If they want to be agressive, get more branding, increase income and so forth - they need that elusive CL. A bunch of them in fact. One cannot buy luck(occasional refs at best), but can players.