Using last month.

Technically in-date, but still needs an update.

Sale 1:

Sold to a bot for 7 keys + 14.11 ref

Seller: https://backpack.tf/profiles/76561198203307876#!/compare/1536192000/1536278400

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1536192000/1536278400

https://backpack.tf/item/4483765211

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

X = 7.5

Sale 2:

Sold for $11.10 on marketplace.tf

https://marketplace.tf/items/30618;5;u3001

https://backpack.tf/item/5350079854

http://manic.tf/keyprice/?at=2018-08-25 - $11.1 / $1.9 = 5.84 keys

X = 6

Sale 3:

Sold to a quickbuyer for 6 keys

Seller: https://backpack.tf/profiles/76561198439478832#!/compare/1536796800/1536883200

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1536796800/1536883200

https://backpack.tf/item/4421878465

X = 6

Other Sales:

https://backpack.tf/item/5657523566 (#1)

https://backpack.tf/item/4421878465 (#2)

https://backpack.tf/item/4416182445 (#3)

https://backpack.tf/item/5374354774 - Not unusual at time of sale

#1

Untraceable

https://backpack.tf/profiles/76561198031352152#!/compare/1535846400/1536969600

https://backpack.tf/profiles/76561198312583107#!/compare/1536364800/1536451200

https://i.imgur.com/ZEzboFC.png - Labelled as 7 keys + 14 ref

#2

Bulk for Miami Nano

https://backpack.tf/profiles/76561198070023446#!/compare/1536796800/1537056000

https://backpack.tf/profiles/76561198439478832#!/compare/1536796800/1536883200

#3

https://backpack.tf/item/4416182445 - untraceable bot purchase

https://i.imgur.com/ZEzboFC.png - Labelled as 7 keys + 15 ref

Additionally, there’s another purchase for ~7.5 keys on bot.tf, but it is untraceable to the point that the item’s history can’t be found.

https://bot.tf/stats/item?def=30618&q=5&ef=Showstopper&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198295122993#!/compare/1536710400/1536796800

https://i.imgur.com/ZEzboFC.png - Labelled as 7 keys + 15.44 ref

Sellers:

https://i.imgur.com/AV6KeGQ.png

https://i.imgur.com/Cg60xE4.png

https://i.imgur.com/tY4aUto.png

https://backpack.tf/item/4421878465 - ~6.56 keys

https://backpack.tf/item/5657523566 - ~9.39 keys

https://backpack.tf/item/4483765211 - 10 keys

https://backpack.tf/item/4416182445 - 12 keys

https://backpack.tf/item/4411510850 - 12 keys

https://backpack.tf/item/6011900550 - 14 keys

https://backpack.tf/item/4412930376 - 14 keys

https://backpack.tf/item/4445071690 - 15 keys

https://backpack.tf/item/5209053526 - 18 keys

https://backpack.tf/item/4978889971 - 18 keys

https://backpack.tf/item/4422952213 - 18 keys

+ 7 more on marketplace for ~7-9 keys worth

https://marketplace.tf/items/30618;5;u3001

Sales at:

6 (2x)

7.5

Going with 6-7.5.