Using last month.

Sales 1-4:

Sold for $52.50, $29.95, $29.94, and $19 on marketplace.tf

https://marketplace.tf/items/1115;5;u3005

https://backpack.tf/item/6424390837 - No outpost

https://backpack.tf/item/6333966465 - No outpost

https://backpack.tf/item/4738775708 - No outpost

Can’t find the last one

http://manicbot.me/keyprice/?at=2018-07-28

29.95/1.99 = ~15.05

29.94/1.99 = ~15.05

http://manicbot.me/keyprice/?at=2018-08-06 - 52.5/1.9 = ~27.63

http://manicbot.me/keyprice/?at=2018-08-15 - 19/1.88 = ~10.11

X = 28

X = 15 (2x)

X = 10

Sale 5:

Sold for 18 keys

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1534118400/1534204800

Buyer: https://backpack.tf/profiles/76561198223191956#!/compare/1534032000/1534204800

https://backpack.tf/item/4119926819

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

X = 18

Sale 6:

Sold for 18 keys

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1534291200/1534377600

Buyer: https://backpack.tf/profiles/76561198117447379#!/compare/1534291200/1534377600

https://backpack.tf/item/3637398385

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

X = 18

Sale 7:

Sold for 15 keys + 2 ref

Seller: https://backpack.tf/profiles/76561198398355965#!/compare/1534291200/1534377600

Buyer: https://backpack.tf/profiles/76561198090407541#!/compare/1534291200/1534377600

https://backpack.tf/item/4479310230

X = 15

Sale 8:

Quicksold for 13 keys

Seller: https://backpack.tf/profiles/76561198349344794#!/compare/1532476800/1532563200

Buyer: https://backpack.tf/profiles/76561198142653871#!/compare/1532304000/1532563200

https://backpack.tf/item/3344359730

X = 13

Sale 9:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198158206236#!/compare/1532390400/1532476800

Buyer: https://backpack.tf/profiles/76561198349344794#!/compare/1532476800/1532563200

https://backpack.tf/item/3344359730

X = 12

Sale 10:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198042540572#!/compare/1534377600/1534550400

Buyer: https://backpack.tf/profiles/76561198125424407#!/compare/1533513600/1534550400

https://backpack.tf/item/3507592706

X = 9

Sellers:

https://i.imgur.com/RteLg6v.png

https://backpack.tf/item/5375349526 - 29

https://backpack.tf/item/3213888640 - 30

https://backpack.tf/item/4540462561 - 55

Sales at:

9

10

12

13

15 (3x)

18 (2x)

28

Buyers at ~12.5, excluding 12 and lower.

13 quicksold, excluding 13.

28 is a large outlier, excluding 28.

Going with 15-18.