Sick range.

Using last 2 months due to lack of sales.

Sales 1 & 2:

Sold for $32.50 and $33.32 on marketplace.tf

https://marketplace.tf/items/1111;5;u3001

http://manic.tf/keyprice/?at=2018-09-04 - $32.50 / $1.88 = 17.29 keys

http://manic.tf/keyprice/?at=2018-07-31 - $33.32 / $1.96 = 17.00 keys

https://backpack.tf/item/5186628068 - No B/O

Couldn't find other item history.

X = 17 (2x)

Sale 3:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198839307425#!/compare/1534032000/1534118400

Buyer?: https://backpack.tf/profiles/76561198307006895#!/compare/1533686400/1534118400

https://backpack.tf/item/7036185508

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

Looks like either the buyer sold their burning pirate bandana for keys and bought both taunts that appear there, or the user that bought the burning pirate bandana is the buyer here, but didn't have their backpack scanned with the item after buying (more likely the case here). Either way, I'm confident that 11 keys is correct here.

X = 11

Other Sales:

https://backpack.tf/item/7108227710 - tradeit

https://backpack.tf/item/5810340622 - tf2mart

https://backpack.tf/item/5753867820 - scammer stuff

https://backpack.tf/item/5113019137 - swap bots

https://backpack.tf/item/6256635738 - tf2mart

Sellers:

https://i.imgur.com/Qm20a5F.png

https://backpack.tf/item/6375072233 - 20 keys

https://backpack.tf/item/5277741952 - 30 keys

And another seller for $28.68 on the SCM:

https://steamcommunity.com/market/listings/440/Unusual%20Taunt%3A%20Skullcracker?filter=showstopper

https://i.imgur.com/qdgfSVd.png - ~11.5 keys

Sales at:

11

17 (2x)

Keeping 11 due to the SCM seller.

Going with 11-17.