Using last month.

Sale 1:

Sold to a bot for 3 keys + 21.33 ref

Seller: https://backpack.tf/profiles/76561198161977280#!/compare/1543104000/1543190400

Buyer: https://backpack.tf/profiles/76561198335791307#!/compare/1543017600/1543104000

https://backpack.tf/item/7044455526

https://i.imgur.com/Nvj3ytO.png - confirmed by bot.tf

X = 3.5

Sale 2:

Sold for 2 keys + 58 ref (~1.5 keys)

Seller: https://backpack.tf/profiles/76561198004290837#!/compare/1543017600/1543104000

Buyer: https://backpack.tf/profiles/76561198272619567#!/compare/1542758400/1543017600

https://backpack.tf/item/5508476579

X = 3.5

Other Sales:

https://backpack.tf/item/5373720417 (#1)

https://backpack.tf/item/5301413088 (#2 & #3)

https://backpack.tf/item/5508476579 (#4-7)

https://backpack.tf/item/6648039904 - scrap

#1

Bulk for Slumber Tipped Lid

https://backpack.tf/profiles/76561198273866103#!/compare/1544572800/1544745600

https://backpack.tf/profiles/76561198103341552#!/compare/1544659200/1544745600

#2

Untraceable

https://backpack.tf/profiles/76561198093501381#!/compare/1543276800/1543363200

https://backpack.tf/profiles/76561198292737956#!/compare/1537401600/1543363200

#3

SCM

https://backpack.tf/profiles/76561198292737956#!/compare/1543622400/1543795200

https://backpack.tf/profiles/76561198087268284#!/compare/1543881600/1544140800

#4

Untraceable

https://backpack.tf/profiles/76561198307761034#!/compare/1542844800/1542931200

https://backpack.tf/profiles/76561198274189745#!/compare/1542844800/1542931200

#5

Bulk for Bubbling Trilby

https://backpack.tf/profiles/76561198274189745#!/compare/1542844800/1542931200

https://backpack.tf/profiles/76561198004290837#!/compare/1542931200/1543017600

#6

Untraceable

https://backpack.tf/profiles/76561198272619567#!/compare/1543017600/1543104000

https://backpack.tf/profiles/76561198423134996#!/compare/1542931200/1543017600

#7

Untraceable

https://backpack.tf/profiles/76561198423134996#!/compare/1543017600/1543104000

https://backpack.tf/profiles/76561198312480435#!/compare/1542931200/1543017600

https://i.imgur.com/Nvj3ytO.png - Labelled as a ~3.5 key purchase.

Sellers:

https://i.imgur.com/mRf96RO.png

https://backpack.tf/item/6470784473 - 4 keys

https://backpack.tf/item/5792765038 - ~4.35 keys

https://backpack.tf/item/6648039904 - ~4.35 keys

https://backpack.tf/item/7044649241 - ~4.35 keys

https://backpack.tf/item/7044455526 - ~4.35 keys

Sales at:

3.5 (2x)

There’s a bot.tf sale at ~4.35 keys that’s barely a month old, but given that the sale is untraceable, and the fact that it was re-sold to the bot for ~3.5 keys, it would be better to ignore that sale. Even if it were valid, there’s currently a seller at 4.

https://backpack.tf/profiles/76561198334598690#!/compare/1542150000/1542236400

https://backpack.tf/profiles/76561198299767615#!/compare/1541980800/1542153600

https://backpack.tf/item/7044649241

Going with 3.5 flat.