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.
however, this value has decreased. that's what's happening right now with Ronaldo. apart from his old age as a football athlete. His performance at Man United is currently also declining.
somehow it is very difficult for Ronaldo to get a club that plays in the Champions League. that's what he's looking for right now. even willing to pay less of course it should be an attraction for the club.
before wanting to leave, I think Ronaldo has to show his ability is still there. and playing at his best for Man United is a great way to attract interest from other big clubs.