I think that Madrid wants De Gea just as much as he wants them. He's the heir apparent to Saint Iker in the Spanish national team, and Madrid clearly want that at club level as well. The simple reality is, they have most of the leverage. They seem comfortable with their goalkeeping options, even with Casilas' exit. They know De Gea, his family, his girlfriend and his agent all want the transfer to happen. They're very well aware that De Gea only has one year left on his deal, and that they can sign him to a pre-contract agreement in six months time. Aside from Ramos, none of their players have angled for a move in the other direction. As far as Ramos is concerned, logic suggests that he's simply been maneuvering for an improved deal - one which he'll likely get. To top it off, De Gea is seemingly badly distracted by all of it, resulting in a drop in his performance level and a major distraction for the club on the eve of the new season.
The only footing United has is the prospect of keeping De Gea for the season and letting him leave for free next summer. That said, Madrid will know that, principles aside, United won't want to lose him for nothing, and certainly won't want this cloud hanging over them all season. I believe they're prepared to wait, rather than overpay (in their minds). Madrid will know that, even if we refuse to sell him, De Gea won't want to risk his place in the national team in advance of the Euros, and that he'll knuckle down and work hard. Quite frankly, even if we parked him in the reserves all season, I really don't think they'd care. De Ge would be the one incurring all the risk by riding out the last year if his contract, not Madrid.
In my mind, the most likely outcome is that Madrid up their offer by a very moderate amount as the transfer window winds down, and we let him go. I can't see us getting more than around 20M pounds for him. I sure hope I'm wrong!