Using last month.

Sale 1:

Sold for $23.97

https://marketplace.tf/items/30919;5;u3003

https://manic.tf/keyprice/?at=2018-10-05 - $23.97 / $1.8 = 13.32 keys

https://backpack.tf/item/6480776847

X = 13

Sale 2:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198317606171#!/compare/1537142400/1537228800

Buyer: https://backpack.tf/profiles/76561198070664972#!/compare/1537142400/1537228800

https://backpack.tf/item/6479776347

X = 11

Sale 3:

Sold for 9 keys to a quickbuyer

Seller: https://backpack.tf/profiles/76561198046921199#!/compare/1538352000/1538438400

Buyer: https://backpack.tf/profiles/76561198397249586#!/compare/1538352000/1538524800

https://backpack.tf/item/6465206740

X = 9

Other Sales:

https://backpack.tf/item/6890761564 (#1)

https://backpack.tf/item/6479044489 (#3-6)

https://backpack.tf/item/6466085527 (#7)

https://backpack.tf/item/6800276595 - to scrap

https://backpack.tf/item/6493300909 - marketplace

https://backpack.tf/item/6488289157 - tf2mart

https://backpack.tf/item/6487042002 - not unusual at the time

#1

Bulk for Neutron Star Gibus

https://backpack.tf/profiles/76561198090460161#!/compare/1538006400/1538179200

https://backpack.tf/profiles/76561198061489334#!/compare/1538092800/1538179200

#2

Sold with keys for Eerie Festivizer

https://backpack.tf/profiles/76561198110248446#!/compare/1537488000/1537574400

https://backpack.tf/profiles/76561198102538699#!/compare/1537488000/1537574400

Way too high if used here.

#3

Bulk for Beams Modest

https://backpack.tf/profiles/76561198082436352#!/compare/1537228800/1537315200

https://backpack.tf/profiles/76561198218497692#!/compare/1537228800/1537315200

#4

Bulk for Peace Sign Rack

https://backpack.tf/profiles/76561198218497692#!/compare/1537401600/1537488000

https://backpack.tf/profiles/76561198136702159#!/compare/1537401600/1537488000

#5

Probably gambled

https://backpack.tf/profiles/76561198136702159#!/compare/1538352000/1538524800

https://backpack.tf/profiles/76561198101300189#!/compare/1538352000/1538438400

#6

Bulk

https://backpack.tf/profiles/76561198101300189#!/compare/1538524800/1538611200

https://backpack.tf/profiles/76561198837822461#!/compare/1538524800/1538611200

#7

Untraceable, probably SCM

https://backpack.tf/profiles/76561198016831741#!/compare/1532131200/1538697600

Sellers:

https://i.imgur.com/p442hdx.png

https://backpack.tf/item/6637016931 - 14 keys

https://backpack.tf/item/6890761564 - 16 keys

https://backpack.tf/item/6722070393 - 30 keys

Sales at:

9

11

13

9 was a single sale to a quickbuyer, with several buyers still at 9. Excluding 9.

Going with 11-13.