The main selling point of the new Yanling planeswalker is the ultimate ability, which lets you draw a card every time you tap an island. That is incredibly strong if you can get there in the course of the match-up.
However, you can't use her second ability right after she comes into play. The only other planeswalker in MtG that had the same problem was Tibalt, who didn't do very well.
Taking this into account can indicate a gradual drop in price, and the only reason for the current price is the novelty factor, which will lose its value over the course of time.