IF it happened this summer, I'd expect Robinho in return immediately.
And I'd hope United push for another player, Sergio Ramos would be awesome. That would really test how much Real want him.
With those 2 involved I don't really think any money is particularly necessary but if we can squeeze another I dunno... £50M out of them I'd be happy.
I'm not sure about Diarra though. He's a great DM, but we already have so many central midfielders. BUT he is a completely different type of midfielder to the ones we have, a real athlete, big strong and always getting into challenges. When playing against the likes of Chelsea it's always useful to have someone like that in the middle of the park but other than in those games I can't really see a use for him.
But other than the 3 mentioned players, I don't think any of Madrid's players are particularly desirable or available at all in the case of Casillas.
If the only player involved in the exchange were Robinho I'd be disappointed with much less than £75M.
If Sergio Ramos is included any amount of cash will be a bonus.
If it's Robinho + Diarra, I want £50M
And if it's Ramos + Diarra, I want around £25M.
That's not too much to ask for gawd's sake!
