Soleil cards are unlike other colector cards. It is an understanding that contractually UR has to resign the card after 6 months of release. With this thought in mind many players buy/save their Soleil cards instead of the normal practice of selling them.
In practice this actually cause high supply with low demand, meaning that they will mostly not be as high as a non Soleil card.
Personaly, I keep about 4 copies of each card just in case it turns Cr.