Doubt it'd be considerable
If is on the rumored 6 or 8 annual net and could be getting either double or less of what he's on, then Real could easily copy the terms until 2025 but probably wouldn't even begin to think about it as don't value him that high
A Perez classic to not pay his defensive players unless they're Spanish and/or captain
Which IMO is a mistake in short term and all parties will pay
Don't think Casemiro himself was even thinking of leaving before 2 days ago when they called. Which granted speaks poorly of him as is clearly either after more guaranteed cash, or simply hates competing for his spot in the 11
Not sure it really speaks poorly of him though. I used to think the same, but think about this way.....he has achieved literally everything he could in Madrid, apart from a treble which you'll never win, but anyway....
He's not from a particularly wealthy background. Single mother working double shifts and everything and Man Utd are offering him double of what he receives in Madrid and he's 30 years old. It's easy to see why he would take it.
Sure he's made a lot of money in his career already, but double the money when he could start slowing down at anytime now. Man's just getting his bag and setting up his family for generations.
Think it's okay to cut players from such backgrounds some slack for their decisions. Especially when you have already won everything already at club level and multiple times too.