Using last 2 months, due to lack of sales here.

The Sale:

Sold for $14.66 on marketplace.tf

https://marketplace.tf/items/30671;5;u3001

https://backpack.tf/item/4405545081 - no outpost

http://manic.tf/keyprice/?at=2018-07-28 - $14.66 / $1.99 = 7.37 keys

X = 7.5

Other Sales:

https://backpack.tf/item/6847785170 (#1 & #2)

https://backpack.tf/item/6683268714 (#3)

https://backpack.tf/item/5927831955 (#4-6)

https://backpack.tf/item/5398497726 (#7)

https://backpack.tf/item/4431564960 (#8)

https://backpack.tf/item/4405541531 (#9-11)

https://backpack.tf/item/6619484850 (#12 & #13)

https://backpack.tf/item/6997110535 (#14-18)

https://backpack.tf/item/7115601811 - scammer/marketplace stuff

https://backpack.tf/item/4722815625 - tf2mart bot stuff

https://backpack.tf/item/4581541681 - tf2mart bot stuff

https://backpack.tf/item/6836573559 - tradeit/bitskins stuff

#1

Might be bulk

https://backpack.tf/profiles/76561198289838885 - "inventory service unavailable"

https://backpack.tf/profiles/76561198159448766#!/compare/1535155200/1536796800

#2

SCM

https://backpack.tf/profiles/76561198159448766#!/compare/1535155200/1536796800

https://backpack.tf/profiles/76561198079784600#!/compare/1536969600/1537056000

#3

Untraceable

https://backpack.tf/profiles/76561198278315300#!/compare/1536710400/1536883200

https://backpack.tf/profiles/76561198397249586#!/compare/1536710400/1536796800

#4

Bulk for Gold Pan

https://backpack.tf/profiles/76561198013245606#!/compare/1536624000/1536710400

https://backpack.tf/profiles/76561198202477376#!/compare/1536624000/1536710400

#5

Untraceable

https://backpack.tf/profiles/76561198202477376#!/compare/1536883200/1536969600

https://backpack.tf/profiles/76561198126910873#!/compare/1536710400/1536969600

#6

Untraceable

https://backpack.tf/profiles/76561198126910873#!/compare/1537228800/1537401600

https://backpack.tf/profiles/76561198110953461#!/compare/1537142400/1537401600

#7

Sold with a bunch of items for Green Confetti Tundra Top

https://backpack.tf/profiles/76561198088870405#!/compare/1537315200/1537401600

https://backpack.tf/profiles/76561198151067911#!/compare/1537315200/1537401600

Too bulky to be used here, would be better to update the Tundra again with this sale.

#8

Sold for 3 unusual taunts that'll end up being over 20 keys

https://backpack.tf/profiles/76561198063640894#!/compare/1535414400/1535500800

https://backpack.tf/profiles/76561198094499038#!/compare/1535155200/1535500800

Way too high here.

#9

Untraceable

https://backpack.tf/profiles/76561198312583107#!/compare/1535241600/1535328000

https://i.imgur.com/xN7dpdK.png - Labelled as 10 keys

#10

Untraceable

https://backpack.tf/profiles/76561198240537922#!/compare/1534723200/1534809600

https://backpack.tf/profiles/76561198165477682#!/compare/1534723200/1534809600

#11

https://backpack.tf/profiles/76561198165477682#!/compare/1534723200/1534809600

https://backpack.tf/profiles/76561198312583107#!/compare/1534636800/1534723200

https://i.imgur.com/xN7dpdK.png - labelled as 4 keys + 18 ref

#12

SCM

https://backpack.tf/profiles/76561198045794663#!/compare/1532476800/1532563200

https://backpack.tf/profiles/76561198189205206#!/compare/1532390400/1533686400

#13

Untraceable

https://backpack.tf/profiles/76561198189205206#!/compare/1533686400/1535760000

https://backpack.tf/profiles/76561198133882561#!/compare/1534636800/1534896000

#14

Bulk for Vintage Lugermorph

https://backpack.tf/profiles/76561198202706932#!/compare/1533168000/1534204800

https://backpack.tf/profiles/76561198843116519#!/compare/1533081600/1533168000

#15

Untraceable

https://backpack.tf/profiles/76561198843116519#!/compare/1533081600/1533168000

https://backpack.tf/profiles/76561198042132655#!/compare/1533168000/1533254400

#16

Sold for Bubbling Defiant Spartan + ~1.5 keys in pure/items

https://backpack.tf/profiles/76561198042132655#!/compare/1533254400/1533340800

https://backpack.tf/profiles/76561198371594889#!/compare/1533168000/1533340800

Too high here.

#17

Untraceable

https://backpack.tf/profiles/76561198371594889#!/compare/1533340800/1533686400

https://backpack.tf/profiles/76561197972349641#!/compare/1533340800/1533427200

Item ID's show 3 keys on the seller's end match, and key histories there as well as the ones that left the buyer's backpack share a common user. However, I can't tell how much was involved here. Excluding.

#18

Untraceable

https://backpack.tf/profiles/76561197972349641#!/compare/1533340800/1533427200

https://backpack.tf/profiles/76561198312583107#!/compare/1533254400/1533340800

Sellers:

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

https://backpack.tf/item/5927831955 - 10 keys

https://backpack.tf/item/5398497726 - 19 keys

https://backpack.tf/item/4722815625 - 20 keys

https://backpack.tf/item/5436217194 - 20 keys

+ 3 sellers ranging from ~8.5-20 keys worth on marketplace.tf

https://marketplace.tf/items/30671;5;u3001

Sales at:

7.5

Going with 7.5 flat.