Using last month.

Sale 1:

Sold for 27 keys

Seller: https://backpack.tf/profiles/76561198306872661#!/compare/1536364800/1536451200

Buyer: https://backpack.tf/profiles/76561198042064091#!/compare/1534982400/1536451200

https://backpack.tf/item/6268343743

X = 27

Sale 2:

Sold for 20 keys

Seller: https://backpack.tf/profiles/76561198216772865#!/compare/1535414400/1535587200

Buyer: https://backpack.tf/profiles/76561198322876605#!/compare/1535414400/1535500800

https://backpack.tf/item/6233587049

X = 20

Sale 3:

Sold for 20 keys

Seller: https://backpack.tf/profiles/76561198264117106#!/compare/1535155200/1535241600

Buyer: https://backpack.tf/profiles/76561198124728984#!/compare/1535155200/1535241600

https://backpack.tf/item/4436670811

X = 20

Sale 4:

Sold for 11 keys + Double-spelled Bill's (5)

No B/O

Seller: https://backpack.tf/profiles/76561198315816977#!/compare/1535760000/1535932800

Buyer: https://backpack.tf/profiles/76561198306872661#!/compare/1535846400/1535932800

Bill's:

https://backpack.tf/profiles/76561198109946397#!/compare/1535587200/1535673600 - geniune pro. ks Widowmaker (~4)

https://backpack.tf/profiles/76561198418740133#!/compare/1532131200/1532217600 - 6 keys

Taking 5 on the spelled Bill's.

X = 16

Other Sales:

https://backpack.tf/item/6268343743 (#1)

https://backpack.tf/item/6233587049 (#2)

https://backpack.tf/item/3924250615 (#3)

https://backpack.tf/item/6856884148 - tf2mart

#1

Untraceable

https://backpack.tf/profiles/76561198200464441#!/compare/1535846400/1535932800

https://backpack.tf/profiles/76561198315816977#!/compare/1535760000/1535932800

#2

Untraceable

https://backpack.tf/profiles/76561198322876605#!/compare/1535500800/1535673600

https://backpack.tf/profiles/76561197962856659#!/compare/1529107200/1536278400

#3

Bulk for Frostbite One-Man Army

https://backpack.tf/profiles/76561198136220673#!/compare/1536624000/1536969600

https://backpack.tf/profiles/76561198069388365#!/compare/1536710400/1537056000

Sellers:

https://i.imgur.com/H4V1S0U.png

https://backpack.tf/item/3924250615 - 31 keys

http://www.tf2outpost.com/trade/29729339 - offers

Sales at:

16

20 (2x)

27

16 is low, excluding 16.

Going with 20-27.