I doubt it's a case of not spending the money because they can afford not to.
I reckon it's more a case of "why should we get a player who has made it quite explicit that he would only join Real Madrid and nobody else" ?
that means even if, hypothetically speaking, Ribery does come to Man Utd, one can't help but wonder did he choose them as a second alternative to Real? and if so, would he go flying to Real in a season's time when Real comes knocking on the door?
the same thing can be said for David Villa as well.