IMO they could pay £200m and it still wouldnt be enough.
No money in the world will get you a better player than Ronaldo, doubt Ribery, Benzema, Alves, Villa would get 25+ goals and how ever many assists if Utd were to sign them.
Furthermore... Barca are going to romp the spanish league again next season so the chances are Real will win very little if anything - they havent got past the 1/4 finals of the champions league for the last 5 years! Bad move for Ronaldo imo. Best of luck to him though, he has done well for Utd.