A while back, I searched for months to complete the full set of 6 super-slayer scrappers. I ended up buying the undead slayer for 40 mil.
Since then, they have changed how slayers work, so generally, the super-slayers do less damage than a normal slayer lowering their value. Since the Undead Super-slayer has no sub-class, I am guessing that the price would still be in that range.
btw... I bought on Atlantic so prices may be inflated beyond what it would go for on another shard.