Using sales since August, due to lack of stable sales in the last month.

Sales 1-3:

Sold for $29.50, $19.39, and $10 on marketplace.tf

https://marketplace.tf/items/30761;5;u3001

https://backpack.tf/item/5377887700 - no outpost, couldn’t find the others

http://manic.tf/keyprice/?at=2018-08-05 - $19.39 / $1.9 = 10.21 keys

http://manic.tf/keyprice/?at=2018-08-28 - $29.5 / $1.88 = 15.69 keys

http://manic.tf/keyprice/?at=2018-09-03 - $10 / $1.89 = 5.29 keys

X = 16

X = 10

X = 5.5

Sale 4:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198363395191#!/compare/1536364800/1536451200

Buyer: https://backpack.tf/profiles/76561198048017265#!/compare/1536192000/1536537600

https://backpack.tf/item/6965899882

X = 7

Other Sales:

https://backpack.tf/item/6965899882 (#1 & #2)

https://backpack.tf/item/4974864357 (#3 & #4)

https://backpack.tf/item/4960138846 (#5-7)

https://backpack.tf/item/6554661179 (#8 & #9)

https://backpack.tf/item/6535356364 - marketplace/swap bot

https://backpack.tf/item/6504013752 - not unusual at time of sale

https://backpack.tf/item/6176065989 - appears to have been transferred between alt/friend accounts

https://backpack.tf/item/5034598652 - also appears to have been transferred between alt/friend accounts

https://backpack.tf/item/4974864357 - swap bot

https://backpack.tf/item/4960378812 - gambled, scammers and swap bot

#1

Untraceable

https://backpack.tf/profiles/76561198334598690#!/compare/1535587200/1535673600

https://backpack.tf/profiles/76561198099136395#!/compare/1535587200/1535673600

https://i.imgur.com/GyTrBHW.png - labelled as 10, only some keys match here though.

#2

bot.tf purchase from a scammer

https://backpack.tf/item/6965899882

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

#3

Untraceable

https://backpack.tf/profiles/76561198311498888#!/compare/1535241600/1535328000

https://backpack.tf/profiles/76561198074002812#!/compare/1534809600/1535328000

#4

https://backpack.tf/profiles/76561198156689043#!/compare/1534896000/1535673600

https://backpack.tf/profiles/76561198311498888#!/compare/1534896000/1537660800

#5

Bulk for Slumber Modest

https://backpack.tf/profiles/76561198354295315#!/compare/1524096000/1535846400

https://backpack.tf/profiles/76561198144776495#!/compare/1535760000/1535846400

#6

Sold for Neutron Star Lil’ Bitey

https://backpack.tf/profiles/76561198144776495#!/compare/1535760000/1535846400

https://backpack.tf/profiles/76561198202995592#!/compare/1535760000/1535846400

#7

Not unusual at time of sale

https://backpack.tf/profiles/76561198301240927#!/compare/1534291200/1534636800

https://backpack.tf/profiles/76561198354295315#!/compare/1524096000/1535846400

#8

Bulk for Aces High Glasgow Great Helm

https://backpack.tf/profiles/76561198152520083#!/compare/1533772800/1533859200

https://backpack.tf/profiles/76561198033378852#!/compare/1533772800/1533859200

#9

Untraceable

https://backpack.tf/profiles/76561198033378852#!/compare/1534204800/1534291200

https://backpack.tf/profiles/76561198003926663#!/compare/1534204800/1534291200

Sellers:

https://i.imgur.com/Jngq5fo.png

https://backpack.tf/item/6504013752- 20 keys

https://backpack.tf/item/4952822733 - 29 keys

And one on marketplace for $22.22 (~12 keys)

https://marketplace.tf/items/30761;5;u3001

Sales at:

5.5

7

10

16

16 looks like an outlier, excluding 16

5.5 is lower than buyers, excluding.

Going with 7-10.