Cristiano is on 12M a year. So if he's being offered 15M a year that is 25% increase. However with the taxes that means he's going from 15M gross to 30M gross which is a 100% increase. If he wants 20M net then that is 40M a year gross which is a 266% increase.
It's a problem for Real. They have to decide what to offer him or consider selling him. He has only 2 years on his contract. If he doesn't renew his value will plummet next season as a year later he will be free. Either way, the reduction in transfer value will be payed by the buying club to Cristiano.
If Monaco do go for him, since they are tax free they would save 100M over a 5 year contract compared to anyone else, so definitely the club that can most afford him.
However, with Falcao at Monaco, Neymar at Barca, Higuain also leaving Madrid, I don't see how they can afford to let him go. I guess they could get Cavani but City and Chelsea also might bid for him.