Sale 1:

Sold for 16 keys + 12 ref

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1551484800/1551571200

Buyer: https://backpack.tf/profiles/76561198062561069#!/compare/1551484800/1551571200

https://backpack.tf/item/277134753

Seller’s end shows two lots of keys/ref from the same buyer in the same group of ID’s, but it’s clear which one belongs to the tyrant’s helm, as the other sold for 14 keys + 8 ref

X = 16

Sale 2:

Sold after a trade hold for 16 keys

Seller: https://backpack.tf/profiles/76561198062561069#!/compare/1551657600/1552089600

Buyer: https://backpack.tf/profiles/76561198019820977#!/compare/1551484800/1552089600

https://backpack.tf/item/277134753

X = 16

Sale 3:

Sold to a quickbuyer for 15 keys + 9 ref

Seller: https://backpack.tf/profiles/76561198173194325#!/compare/1549497600/1549843200

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1549411200/1549497600

https://backpack.tf/item/277134753

X = 15

Other Sales:

https://backpack.tf/item/6059188956 - nothing usable

Sellers:

https://i.imgur.com/BEfvGkN.png

https://backpack.tf/item/277115576 - 20 keys

Sales at:

15

16 (2x)

15 went to a quickbuyer, excluding.

Going with 16 flat.