Being able to control 4 seats is so valuable to RBR that I can't imagine anyone offering an amount that RB finds suitable. I think the sister team is here for the long haul.
I'd imagine if you're a young driver, you want to choose Red Bull's academy because it gives you a much higher chance of actually getting an F1 seat compared to Ferrari, Mercedes, etc. This means Red Bull basically gets first pick on every up and coming driver.
And it shows too. Nearly half the grid has come out of the RBYDA. There’s just such a higher chance of getting into F1 that it’s hard to pass up, even if they only get short stints to prove themselves.