Cristiano Ronaldo's contract with Real Madrid ends in 2018, but Paris Saint-Germain will go after him next summer, according to reports from L'Equipe. That will probably be the last chance for Real Madrid to cash some big money on Ronaldo, so maybe Los Blancos will enter the negotiations if PSG show real commitment on a potential transfer.
Furthermore, AS are reporting today that the French club let Real Madrid know that they will make Cristiano Ronaldo a big offer if Florentino Pérez and his board try to sign Marco Verratti. The French club is trying to keep things under control and now Real Madrid will probably stay put and try to sign another midfielder.
Either way, Los Blancos will probably listen to what PSG have to offer next summer. Cristiano Ronaldo will be 31 years old then and it would probably be a good time to entirely rebuild the squad around somebody else while PSG spend a lot of money on Ronaldo.