Using last month.

Sales 1 & 2:

Sold for $14.99 twice on marketplace.tf

https://marketplace.tf/items/30572;5;u3001

https://backpack.tf/item/4160871406 - No outpost (other sales were not unusual at the time), can’t find the other history

http://manic.tf/keyprice/?at=2018-09-07 - $14.99 / $1.88 = 7.97 keys

http://manic.tf/keyprice/?at=2018-09-09 - $14.99 / $1.87 = 8.02 keys

X = 8 (2x)

Sale 3:

Sold for 5 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198262056688#!/compare/1534636800/1534809600

Buyer: https://backpack.tf/profiles/76561198079413347#!/compare/1534550400/1534636800

https://backpack.tf/item/3843171830

X = 5.5

Sale 4:

Sold to a bot for 4 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198105247417#!/compare/1536192000/1536278400

Buyer: https://backpack.tf/profiles/76561198363395191#!/compare/1536192000/1536278400

https://backpack.tf/item/5534231099

X = 4.5

Other Sales:

https://backpack.tf/item/3877474175 (#1 & #2)

https://backpack.tf/item/3631261225 (#3 & #4)

https://backpack.tf/item/3869595609 - Scammers & Spec. KS

#1

Untraceable

https://backpack.tf/profiles/76561198135982067#!/compare/1536624000/1537056000

https://backpack.tf/profiles/76561198363395191#!/compare/1536969600/1537056000

#2

Untraceable

https://backpack.tf/profiles/76561198363395191#!/compare/1537142400/1537228800

https://backpack.tf/profiles/76561198070664972#!/compare/1537056000/1537142400

#3

Bulk for stuff

https://backpack.tf/profiles/76561198092497296#!/compare/1535760000/1537228800

https://backpack.tf/profiles/76561198202186637#!/compare/1535587200/1535760000

#4

Bulk for more stuff

https://backpack.tf/profiles/76561198202186637#!/compare/1535932800/1536019200

https://backpack.tf/profiles/76561198251888442#!/compare/1535932800/1536019200

Sellers:

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

https://backpack.tf/item/3671843018 - 8 keys

https://backpack.tf/item/6270000756 - 9 keys

https://backpack.tf/item/4983838497 - 9 keys

Sales at:

4.5

5.5

8 (2x)

4.5 looks kinda low, and doesn’t appear to be a common trading point. Excluding 4.5

Nothing in the last month on bot.tf.

Going with 5.5-8.