Ronaldo's market value has decreased by 10 million to become 20 million euros in three months.
The main reason should be his not having much chance after starting to do his best to leave Manchester United. He hasn't played properly in the starting eleven for some time until the Europa League matches. I still don't expect him to be in the starting eleven in their next Premier League match. Actually his market value would still decrease slowly because of his age above all else. But his ineffective performance is helping his value to be lower quickly. Depending on his recent performance he has a less chance to find himself a club that is in the Champions League. So he needs to show a better performance to help Manchester United be a part of it next season.
Ronaldo's main challenge started when he wants to leave Manchester United by all means because he wanted to play in the Champions league. His decision affected his relationship with the club and even the fans negatively. His market value would continue to reduce if he doesn't improve his performance, which might be very difficult considering his age and less playing time.
Although he is the second choice in Barcelona squad, he can play with quality players. He also seems suitable with Xavi tactics, so he feels comfortable staying in Barcelona. While if he moves to Man United or Chelsea, he will be the main player but may not be suitable with the tactics of the coach on these clubs. He also may feel uncomfortable with a bad situation the new clubs, especially Man United or Chelsea. These clubs are in a very bad situation this season.
Frenkie de Jong is a young talented player that any club or coach would be very glad to have in the team. But due to the financial challenge Barcelona had, they wanted to sell him to raise funds and also reduce wage bills. And the club had already made the necessary adjustments to cover up his absence, but he insisted to stay in the club. Now he has to endure being a substitute which would definitely affect his performance and future value. Sometimes it is better to leave a club that is not really in need of your service for even a less prestigious club that is desperately in need of you. This is because a good player deserves more playing time to improve his performance and market value.