I assume the price is based on what I paid for mine. For those looking, I can say this is a perfectly reasonable price (at least with current rarity) and certainly inline with the only other previous sale (me lol).
EDIT: I think my comment needs more context. This item is very much a collectors item, or at the least for those interested in fashion. Most of you won't be interested in this item as it lacks practical application (perfectly reasonable). But I think I can lean on my own experience in collecting, as owner of a number of collections, some of which are the most extensive in the game.
If MA release the same cloak for next Halloween Mayhem, then this will not be worth the asking price. If kept to this Mayhem, and total drops stay roughly below 20, I'd say it stays reasonably priced. So, there is certainly an element of gamble at this price point which is worth weighing up.
I compare this item similarly to another in my collection, the Deer Horns. From the umbranoid event, I know of 3 other people with it, and current valuation puts it around 4.5K.
Ultimately the market decides what it is worth but I know a few collectors who would snatch this up. GL with sale!
EDIT 2: This is my personal opinion, not a fact. Consider it, but don't blindly listen to it