Any clues?

It comes from a which was sold for 250 csc in the CS when it was available.

Not seeing any boxes on market either. Any idea what these uncommon items generally sell for?

They aren't that uncommon imo. It seems here that priced it at 5.3m but it did not sell (From 2016)

In 2015, sold theirs for 3m as you can see here. For this necklace, i wouldn't go over 6m unless you're patient to try to get around 250 csc.

That never was a desirable item of the set, so 5M seems like a fair price