Using last month.

Sale 1:

Sold with skin (~1) for Electric Hat Protector HFoHA (9)

No outpost

Seller: https://backpack.tf/profiles/76561198302161698#!/compare/1535414400/1535587200

Buyer: https://backpack.tf/profiles/76561198076191312#!/compare/1535587200/1535673600

https://backpack.tf/item/6253203283

X = 8

Sales 2 & 3:

https://marketplace.tf/items/1182;5;u3001

https://backpack.tf/item/6574615631 - no outpost

Can’t find the other.

http://manic.tf/keyprice/?at=2018-08-19 - $13.99 / $1.9 = 7.36 keys

http://manic.tf/keyprice/?at=2018-08-24 - $10 / $1.89 = 5.29 keys

X = 7.5

X = 5.5

Sale 4:

Sold for 6 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198076191312#!/compare/1536192000/1536537600

Buyer: https://backpack.tf/profiles/76561198403552512#!/compare/1536192000/1536278400

https://backpack.tf/item/6253203283

X = 6.5

Sale 5:

Sold for 6 keys + 8 ref

Seller: https://backpack.tf/profiles/76561198833317554#!/compare/1534636800/1534723200

Buyer: https://backpack.tf/profiles/76561198238769318#!/compare/1534291200/1534723200

https://backpack.tf/item/6315071777

X = 6

Other Sales:

https://backpack.tf/item/7049599479 (#1)

https://backpack.tf/item/6385220952 (#2)

https://backpack.tf/item/6321742075 (#3, wasn’t unusual at the time of the other sales)

https://backpack.tf/item/6262640871 (#4)

https://backpack.tf/item/6261786584 (#5)

https://backpack.tf/item/6252925330 (#6 & #7)

https://backpack.tf/item/6315071777 (#8)

https://backpack.tf/item/6252782100 - wasn’t unusual at the time of the sales

https://backpack.tf/item/7012783364 - scammed

#1

Untraceable

https://backpack.tf/profiles/76561198081907770#!/compare/1536364800/1536451200

https://backpack.tf/profiles/76561198312583107#!/compare/1536278400/1536364800

https://i.imgur.com/d9G94jN.png - Labelled as 5 keys and 6 ref

#2

no

https://backpack.tf/profiles/76561198164200492#!/compare/1536451200/1536710400

https://backpack.tf/profiles/76561198839258291#!/compare/1536624000/1536710400

#3

Untraceable

https://backpack.tf/profiles/76561198047510806#!/compare/1535587200/1535673600

https://backpack.tf/profiles/76561197972383519#!/compare/1535587200/1535673600

#4

SCM

https://backpack.tf/profiles/76561198055694560#!/compare/1534118400/1534723200

https://backpack.tf/profiles/76561198163426490#!/compare/1534896000/1535414400

#5

Bulk

https://backpack.tf/profiles/76561198065409820#!/compare/1535155200/1535414400

https://backpack.tf/profiles/76561198005909532#!/compare/1535414400/1536278400

#6

Untraceable

https://backpack.tf/profiles/76561198853954134#!/compare/1536537600/1536624000

https://backpack.tf/profiles/76561198827701520#!/compare/1536451200/1536537600

#7

Untraceable

https://backpack.tf/profiles/76561198827701520#!/compare/1536624000/1536710400

https://backpack.tf/profiles/76561198093501381#!/compare/1536624000/1536710400

#8

Untraceable

https://backpack.tf/profiles/76561198135508009#!/compare/1533600000/1536796800

https://backpack.tf/profiles/76561198833317554#!/compare/1534464000/1536537600

https://i.imgur.com/d9G94jN.png - labelled as 5 keys and 8 ref

Sellers:

https://i.imgur.com/8NfsBwp.png

https://backpack.tf/item/6252811872 - ~6.73 keys

https://backpack.tf/item/7049599479 - 7 keys

https://backpack.tf/item/6252976073 - 10 keys

https://backpack.tf/item/6369852469 - 11 keys

https://backpack.tf/item/6291717184 - 11 keys

Sales at:

5.5

6

6.5

7.5

8

Excluding 7.5 and 8 due to the 2 sellers for cheaper.

Going with 5.5-6.5.