Using last month.

Sale 1:

Sold for $28 on marketplace.tf

https://marketplace.tf/items/1111;5;u3007

28/1.97 = ~14.2

X = 14

Sale 2:

Quicksold for 7 keys + 13 ref

Seller: https://backpack.tf/profiles/76561198028320526#!/compare/1524441600/1524528000

Buyer: https://backpack.tf/profiles/76561198066105037#!/compare/1524441600/1524528000

X = 7.5

Sale 3:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1525737600/1525824000

Buyer: https://backpack.tf/profiles/76561198243596364#!/compare/1525737600/1525824000

X = 7

Sale 4:

Quicksold for 5 keys

Seller: https://backpack.tf/profiles/76561198340015255#!/compare/1523750400/1523836800

Buyer: https://backpack.tf/profiles/76561198139051069#!/compare/1523750400/1523836800

X = 5

Sale 5:

Bulk for Misty Skull Football Helmet

Seller: https://backpack.tf/profiles/76561198026941076#!/compare/1524355200/1524441600

Buyer: https://backpack.tf/profiles/76561198028320526#!/compare/1524355200/1524441600

Sellers:

~12.2 keys:

https://i.imgur.com/uWXx9xX.png

https://backpack.tf/item/2759436053

13 keys:

https://i.imgur.com/p3xkumP.png

https://backpack.tf/item/2760731942

14 keys:

https://i.imgur.com/R7a5KJo.png

https://backpack.tf/item/3791129775

50 keys:

https://i.imgur.com/K7Igk77.png

https://backpack.tf/item/2992185800

51 keys:

https://i.imgur.com/kGimEG1.png

https://backpack.tf/item/5369086113

Offers:

http://www.tf2outpost.com/trade/29647481

http://www.tf2outpost.com/trade/29628629

Sales at:

5

7

7.5

14

https://i.imgur.com/rZGwuS2.png - Buyers at 5+ keys, excluding 5.

Excluding 14, given the 2-month old seller at 14 keys.

https://backpack.tf/item/2782388019 - 7.5 may have quicksold, but the sale at 7 took at least a week. Should be fine to disregard the “quicksell” part of the 7.5 key sale.

Going with 7-7.5.