Navas isn't a Real outcast because of his quality, he's an outcast because he isn't Spanish. Regardless of how good he is or how well he plays, he's always going to play second fiddle if there's a half-decent Spanish goalkeeper around. He may not want to leave Real, but he'd be stupid to stick around if they get DDG.
As for us dropping our valuation, if half the rumours are true this is only the second bid that Real have made for DDG. By leaving it so late they're forcing our hand and forcing us to make a choice. It's not as straightforward as simply holding on to him to spite Real. If we keep him and he can't get his head straight, we'll have lost £20m, he'll have lost a year (at least) from his career and Real will get him for free next summer. Only one winner there and it isn't us. If we let him go, we at least get some money for him, he'll probably be first choice for his country in the Euros and Real get their great new Spanish figurehead goalkeeper. The only potential win we get is if he stays, plays a blinder and somehow decides that we're right for him after all (i.e. Edurne leaves him). The chances of that are exceedingly unlikely. As much as I want him to continue as our goalkeeper, there's not much we can do. We're in a poor position and to be frank, I'll be happy when this saga is finally over.