Using sales from the last suggestion onwards.

Sale 1:

Sold for 8 keys + 30 ref

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1551830400/1551916800

Buyer: https://backpack.tf/profiles/76561198078496850#!/compare/1551830400/1551916800

https://backpack.tf/item/2858557135

X = 8.5

Sale 2:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1549756800/1549843200

Buyer: https://backpack.tf/profiles/76561198287152881#!/compare/1549756800/1549843200

https://backpack.tf/item/3852509629

X = 8

Marketplace Sales:

https://marketplace.tf/items/1115;5;u3009

https://manic.tf/keyprice/?at=2019-01-28 - $22.22 / $1.78 = 12.48 keys

https://manic.tf/keyprice/?at=2019-02-26 - $18 / $1.79 = 10.06 keys

X = 12

X = 10

Other Sales:

https://backpack.tf/item/3206006651 (#1)

https://backpack.tf/item/6124963673 - scrap

#1

Untraceable

https://backpack.tf/profiles/76561198331219999#!/compare/1548547200/1550793600

https://backpack.tf/profiles/76561198075578532#!/compare/1539561600/1550448000

Sales at:

8

8.5

10

12

12 is way too high given how long it took for the 8.5 key sale to sell (worth noting that whattie bot was selling one for around the same price before getting perma’d). Additionally, the sale at 12 was based off a marketplace sale (which isn’t a good sign for suggestions like this), and there are sellers for a fair bit cheaper than the $22.22 sale on marketplace right now.

Going with 8-10.