Using sales from December onwards.

Sale 1:

Sold for 7 keys + 30 ref

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1545177600/1545264000

Buyer: https://backpack.tf/profiles/76561198056176130#!/compare/1545264000/1545350400

https://backpack.tf/item/6527969871

https://i.imgur.com/Y8shEB4.png - bot.tf confirms.

X = 7.5

Sale 2:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198126887938#!/compare/1545523200/1545782400

Buyer: https://backpack.tf/profiles/76561198134819839#!/compare/1545696000/1545782400

https://backpack.tf/item/6459767627

X = 7

Sale 3:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1544918400/1545004800

Buyer: https://backpack.tf/profiles/76561198374144968#!/compare/1544745600/1545091200

https://backpack.tf/item/4255219420

X = 7

Sale 4:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198297905284#!/compare/1544227200/1544313600

Buyer: https://backpack.tf/profiles/76561197980239391#!/compare/1544313600/1544659200

https://backpack.tf/item/6112681509

X = 6

Marketplace sales:

Sold for $8.60 & $13.75

https://marketplace.tf/items/30673;5;u3003

https://manic.tf/keyprice/?at=2018-12-02 - $8.6 / $1.71 = 5.03 keys

https://manic.tf/keyprice/?at=2018-12-24 - $13.75 / $1.72 = 7.99 keys

X = 8

X = 5

Other Sales:

https://backpack.tf/item/6459767627 (#1)

https://backpack.tf/item/6183748657 (#2 & #3)

https://backpack.tf/item/6112681509 (#4)

https://backpack.tf/item/4321803830 (#5 & #6)

https://backpack.tf/item/7062966120 - scrap

https://backpack.tf/item/7026917904 - gambled/bots

https://backpack.tf/item/6512172356 - tradeit

https://backpack.tf/item/4302513782 - mannco

https://backpack.tf/item/4280453401 - tf2mart

https://backpack.tf/item/4259886875 - marketplace

#1

Bulk for Darkblaze Crocleather Slouch

https://backpack.tf/profiles/76561198145683123#!/compare/1545436800/1545523200

https://backpack.tf/profiles/76561198126887938#!/compare/1545177600/1545523200

#2

Untraceable

https://backpack.tf/profiles/76561198041524603#!/compare/1543622400/1543708800

https://backpack.tf/profiles/76561198065400580#!/compare/1543622400/1543708800

#3

Untraceable

https://backpack.tf/profiles/76561198065400580#!/compare/1543968000/1544054400

https://backpack.tf/profiles/76561198079294342#!/compare/1543536000/1544227200

#4

Bulk for Scorching Head Hedge

https://backpack.tf/profiles/76561197980239391#!/compare/1545868800/1545955200

https://backpack.tf/profiles/76561198394965271#!/compare/1545868800/1545955200

#5

Untraceable

https://backpack.tf/profiles/76561198384823844#!/compare/1543708800/1543795200

https://backpack.tf/profiles/76561198130471936#!/compare/1543708800/1543795200

#6

Sold for Roaring Rockets Box Trot + ~1 key in items

https://backpack.tf/profiles/76561198130471936#!/compare/1543881600/1543968000

https://backpack.tf/profiles/76561198174799574#!/compare/1543881600/1543968000

Can be used on the Box Trot.

Sellers:

https://i.imgur.com/H9w5LMH.png

https://backpack.tf/item/4259886875 - 9 keys

https://backpack.tf/item/4266118100 - 10 keys

And another on scrap for ~13 keys

Sales at:

5

6

7 (2x)

7.5

8

https://i.imgur.com/QdtSSWt.png - Buyer at 5 keys, excluding 5.

Going with 6-8.