The property itself is very rare and pretty useful, notably for Arcane Oozes in Shedaklah (one of the Abyss realms) - unfortunately it does not provide immunity from Mageripper Swarms in Limbo IIRC, which is the other place that immunity is greatly desired.
Usually the only options for Dispel immunity are the lowbie set helm (which requires Cleric or UMD), or set rings from Limbo or Aboleths which basically offer naked immunity. This item is also basically naked, the potential issue is that the amulet slot is usually used for AC, while rings are usually quite suitable for situational swapping.
It has value for two groups of people: 1) Characters that don't require a boost to natural armor (eg: Staffmasters, Wizards, Druids). 2) For use as a swap item for players who lack both the set rings and ability to use the set helms.
In short, it's not a godly item but it has some value and a niche purpose, so it could fetch a good price from someone who has a fitting toon. There is no 'market value' for something like this, so I can't give you a ballpark figure.