It's hard to evaluate Ugin, the Ineffable since his abilities look cool at first sight, but then you realize that no colorless deck in Constructed would want to maindeck it.
It's obviously very good for Limited, but that format never had any impact on the financial side of Magic: The Gathering.
Tron archetypes will most likely never play it due to availability of a huge pool of mana, which makes Ugin's passive redundant. While for other colorless staple archetypes this planeswalker would simply be too expensive for six mana.
Of course, it could find its place in Standard, and if that's the case, then the price will stabilize at its current level. Otherwise, expect a significant drop.