Using last month

Sales 1 & 2:

Sold for $36.99 and $32.19 on Marketplace.tf

https://marketplace.tf/items/167;5;u3010

36.99/1.98 = ~18.7

32.19/1.97 = ~16.3

X = 19

X = 16

Sale 3

Sold for 12 keys + 20 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523577600/1523664000

Buyer: https://backpack.tf/profiles/76561198082684654#!/compare/1523577600/1523664000

Bot.tf confirms: https://i.imgur.com/PBQsoRk.png

X = 13

Sale 4

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198152485272#!/compare/1525046400/1525305600

Buyer: https://backpack.tf/profiles/76561198129197811#!/compare/1525219200/1525305600

X = 10

Sale 5

Sold for 9 keys + 20 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524873600/1524960000

Buyer: https://backpack.tf/profiles/76561198102713495#!/compare/1524787200/1524960000

Bot.tf confirms: https://i.imgur.com/PBQsoRk.png

Ref appears on the seller's end, keys appear on the buyer's end.

X = 10 (Rounding to the nearest integer as the average of all sales is higher than 10)

Sellers:

10 keys:

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

https://backpack.tf/item/1830977454

20 keys:

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

https://backpack.tf/item/1754315473

Sales at:

10 (2x)

13

16

19

The marketplace.tf sales (16 and 19) are gonna be large outliers. Both sales occurred before the sellers for cheaper appeared, and all the pure sales indicate a much lower selling point.

13 also seems pretty high at the moment, but we can't necessarily ignore every sale above 10 here, so I'll keep it in the range.

Going with 10-13.