£45m plus a top class player (and by that I mean Snejder or Ramos) would be what I'd be pushing for.
Tbh, I'm not big on the idea of Robinho plus cash. I think we could go out with a £20m cheque and get ourselves someone just as good, whereas I doubt we'd be able to get ourselves a better attacking midfielder or right back than Sneijder or Ramos for that amount.
I think if we got Sneijder, he'd be the "replacement", meaning we'd be changing our style quite a bit, as he's the passer where Ronaldo's the dribbler. I may be captivated by the Dutch style atm, but I'm dreming of a 4231/433 with Berbs up top, Rooney and Tevez off him, with Sneider in the gap ahead of the two midfielders. Only ponderings ye, but you never know
My view on this whole thing is that we really do hold all the cards. Keeping Ronaldo would be a very good acheivement, and we'd be able to put the last couple of pieces into this puzzle, whereas selling him we'd have the cash to pump into a pretty big overall improvement in the squad.