Whilst it's a lot of money for Juventus to pay, they'll make triple that in shirt sales, and other deals associated with having Ronaldo there. It's quite shrewd business on all parties involved. Real will now either buy Hazard or Neymar, and like Courtois.
Fair do's to Ronaldo, he's achieved everything at Real, so he might as well move on and try another challenge before retiring. He likely feels the slower pace of the Italian game will allow him to use his trickery more, or he feels he can exploit the defenders with his pace. And it was either go to another country or come back to the PL with Man Utd, and I very much doubt that they'd sanction a 100m move for a 33 yr old.