Ronaldo's market value has decreased by 10 million to become 20 million euros in three months.
As others have already said in this thread, the market value that transfermarket is defining is not really a hard number that is valid for everyone that is involved in football. It is just a value that this site is setting following their own rules and parameters. Other sites are doing the same and may have different values for each player.
That being said, it is pretty obvious that the market value of Cristiano Ronaldo will slowly but surely become lower and lower because he is already 37 years old. I would say 20 M€ for a player of his age is still an excellent value. Many good players never had such a high value even in their prime.
Actually, things like this are not too strange if we look at his current age and actually this value for a 37-year-old player is still very large and it is possible that other players of the same age will not get such a market value.
I'm actually curious when he leaves Manchester United whether his salary will decrease too or is it still the same.