Using last month.

Sale 1:

Sold with a key for Smoking Antarctic Eyewear (14)

Seller: https://backpack.tf/profiles/76561198077987068#!/compare/1539129600/1539216000

Buyer: https://backpack.tf/profiles/76561198159383930#!/compare/1539216000/1539302400

https://backpack.tf/item/5016654117

X = 14

Sale 2:

Sold for Aus Grenade Launcher (~13)

Seller: https://backpack.tf/profiles/76561198388578474#!/compare/1539129600/1539302400

Buyer: https://backpack.tf/profiles/76561198419228916#!/compare/1538784000/1539302400

https://backpack.tf/item/3658139307

X = 13

Sales 3 & 4:

Sold for $10.06 & $20.38

https://marketplace.tf/items/1111;5;u3003

https://manic.tf/keyprice/?at=2018-10-06 - $20.38 / $1.81 = 11.26 keys

$10 sale is gonna be low, excluding.

X = 11

Sale 5:

Sold for Peace Sign Capotain (Suggestion at 9.25) + Dueling Banjo (~1)

Seller: https://backpack.tf/profiles/76561198226262240#!/compare/1537574400/1537660800

Buyer: https://backpack.tf/profiles/76561198202998823#!/compare/1537574400/1537660800

Banjo appears to have been selling for less than 1.25 keys at the time of the sale:

https://backpack.tf/suggestion/5bb052cf44325a15ff1990f8

X = 10

Sale 6:

Sold for Dead Presidents Cyborg (Suggestion at 9) + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198092430138#!/compare/1539129600/1539216000

Buyer: https://backpack.tf/profiles/76561198077987068#!/compare/1539129600/1539216000

https://backpack.tf/item/5016654117

X = 9.5

Sale 7:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1538524800/1538611200

Buyer: https://backpack.tf/profiles/76561198394457682#!/compare/1538438400/1538611200

https://backpack.tf/item/2777204453

X = 9

Sale 8:

Sold for 8 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198147006723#!/compare/1538265600/1538438400

Buyer: https://backpack.tf/profiles/76561198388578474#!/compare/1538265600/1538438400

X = 8.5

Sale 9:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198165802400#!/compare/1537228800/1537315200

Buyer: https://backpack.tf/profiles/76561198226262240#!/compare/1537056000/1537315200

https://backpack.tf/item/3509761373

X = 8

Sale 10:

Sold for 7 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198094878771#!/compare/1539475200/1539561600

Buyer: https://backpack.tf/profiles/76561198403630009#!/compare/1539388800/1539475200

https://backpack.tf/item/5016654117

X = 7.5

Sale 11:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198159383930#!/compare/1539388800/1539475200

Buyer: https://backpack.tf/profiles/76561198178301947#!/compare/1539388800/1539475200

https://backpack.tf/item/5016654117

X = 7

Sale 12:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198131658546#!/compare/1538006400/1538092800

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1538006400/1538092800

https://backpack.tf/item/2777204453

X = 7

Other Sales:

https://backpack.tf/item/6926805003 (#1)

https://backpack.tf/item/5016654117 (#2 & #3)

https://backpack.tf/item/3658139307 (#4)

https://backpack.tf/item/3509761373 (#5)

https://backpack.tf/item/3401520677 - tf2mart

https://backpack.tf/item/5901568238 - tradeit

https://backpack.tf/item/4730188374 - not unusual at the time

https://backpack.tf/item/4013132094 - mannco

https://backpack.tf/item/3430804318 - tf2mart

https://backpack.tf/item/3170708839 - scrap

#1

Untraceable

https://backpack.tf/profiles/76561198452735539#!/compare/1539302400/1539734400

https://backpack.tf/profiles/76561198363395191#!/compare/1539302400/1539388800

#2

Sold with some amount of keys for Vivid Ol’ Geezer

https://backpack.tf/profiles/76561198178301947#!/compare/1539388800/1539475200

https://backpack.tf/profiles/76561198094878771#!/compare/1539388800/1539475200

#3

Untraceable

https://backpack.tf/profiles/76561198094878771#!/compare/1539475200/1539561600

https://backpack.tf/profiles/76561198208823517#!/compare/1539388800/1539475200

#4

Untraceable

https://backpack.tf/profiles/76561198419228916#!/compare/1538784000/1539302400

https://backpack.tf/profiles/76561198830830717#!/compare/1539216000/1539302400

#5

Untraceable

https://backpack.tf/profiles/76561198202998823#!/compare/1537833600/1538179200

https://backpack.tf/profiles/76561198155110999#!/compare/1537660800/1537747200

Dead Presidents Cyborg:

https://backpack.tf/suggestion/5bc6e56344325a7ad91c6d42

Circling Peace Sign Carouser’s Capotain:

https://backpack.tf/suggestion/5bc6ee0344325a7df35f4149

Sellers:

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

https://backpack.tf/item/6926805003 - 10 keys

https://backpack.tf/item/5016654117 - 10 keys

https://backpack.tf/item/6897755452 - 13 keys

https://backpack.tf/item/3937303992 - 15 keys

https://backpack.tf/item/4090916283 - 25 keys

Sales at:

7 (2x)

7.5

8

8.5

9

9.5

10 (2x)

11

13

14

13 and 14 are outliers, excluding.

https://i.imgur.com/WvfclB8.png - many buyers higher than 7 keys, excluding 7. Will keep 7.5, however.

Going with 7.5-11.