Re-suggesting, using sales from the last suggestion onwards.

Sale 1:

Sold for on marketplace.tf

https://marketplace.tf/items/30621;5;u3005

http://manic.tf/keyprice/?at=2018-08-27 - $29.99 / $1.89 = 15.87 keys

https://backpack.tf/item/4217362702

http://www.tf2outpost.com/trade/29539937 - B/O of 40

X = 16

Sale 2:

Sold for Blizzardy Storm Pyro’s Beanie (Suggestion at 12)

Seller: https://backpack.tf/profiles/76561198106441454 - backpack isn’t loading, can’t get compare link

Buyer: https://backpack.tf/profiles/76561198092489052#!/compare/1535587200/1535673600

https://backpack.tf/item/4254470743

X = 12

Sale 3:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1534464000/1534550400

Buyer: https://backpack.tf/profiles/76561198133868529#!/compare/1534464000/1534636800

https://backpack.tf/item/4254470743

https://i.imgur.com/ysC2MHN.png

Last set of keys on the seller’s end. Near ID shows a completely different set of keys, most likely because the buyer modified it, changing its ID around the same time the seller obtained the set of keys that match. The buyer obtaining it at the same time as other items are obtained support the modification part of my claim.

X = 11

Sale 4:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1534204800/1534291200

Buyer: https://backpack.tf/profiles/76561198111123084#!/compare/1522195200/1534464000

https://backpack.tf/item/5802558026

https://i.imgur.com/ysC2MHN.png

X = 10

Blizzardy Beanie:

https://backpack.tf/suggestion/5b8d2f9a44325a776a248457

Sellers:

https://gyazo.com/6646a8c9814c0f7ae39339a5b54e7670

https://gyazo.com/fcc549b66b4dff2e3fc23a9b5ee10a9b

https://backpack.tf/item/4441617174 - 18 keys

https://backpack.tf/item/4249921350 - 19 keys

https://backpack.tf/item/4439487552 - 25 keys

https://backpack.tf/item/4317866301 - 27 keys

https://backpack.tf/item/5228677438 - 27 keys

https://backpack.tf/item/5904370449 - 71 keys

Sales at:

10

11

12

16

16 is an outlier, excluding 16.

Going with 10-12.