Using last month.

The Sale:

Sold to a quickbuyer for 4 keys

Seller: https://backpack.tf/profiles/76561198811693329#!/compare/1563235200/1563321600

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1563235200/1563321600

https://backpack.tf/item/6284134782

X = 4

Marketplace Sales:

https://marketplace.tf/items/tf2/1183;5;u3006

$11, Jul 27th

$9.70, Aug 4th

$9.50, Aug 4th

$9.70, Aug 5th

https://manic.tf/keyprice/?at=2019-07-27 - $11 / $1.78 = 6.18 keys

https://manic.tf/keyprice/?at=2019-08-04 - $9.7 / $1.84 = 5.27 keys

https://manic.tf/keyprice/?at=2019-08-04 - $9.5 / $1.84 = 5.16 keys

https://manic.tf/keyprice/?at=2019-08-05 - $9.7 / $1.83 = 5.30 keys

X = 6

X = 5.5 (2x)

X = 5

Other Sales:

https://backpack.tf/item/6644576396 (#1)

https://backpack.tf/item/6257675424 (#2 & #3)

https://backpack.tf/item/6252775691 - scrap

https://backpack.tf/item/6366275253 - marketplace

https://backpack.tf/item/6291884713 - marketplace

#1

Backpack purchase

https://backpack.tf/profiles/76561198065854668#!/compare/1562630400/1562716800

https://backpack.tf/profiles/76561197994697258#!/compare/1562630400/1562716800

#2

Untraceable

https://backpack.tf/profiles/76561198403055175#!/compare/1562889600/1562976000

https://backpack.tf/profiles/76561198363395191#!/compare/1562889600/1562976000

#3

SCM

https://backpack.tf/profiles/76561198363395191#!/compare/1562889600/1562976000

https://backpack.tf/profiles/76561198212034900#!/compare/1563148800/1563753600

#4

Untraceable

https://backpack.tf/profiles/76561198150908404#!/compare/1561680000/1564099200

https://backpack.tf/profiles/76561198305676157#!/compare/1564012800/1564099200

https://i.imgur.com/2ePgiWn.png - labelled as a 3 key + 14 ref purchase (~3.5 keys)

Sellers:

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

https://backpack.tf/item/6284134782 - ~4.7 keys

https://backpack.tf/item/6746324000 - ~4.8 keys

https://backpack.tf/item/6331138329 - ~6.5 keys

https://backpack.tf/item/6868403047 - 8 keys

https://backpack.tf/item/6483599541 - 8 keys

Sales at:

4

5

5.5 (2x)

6

Current highest buyers are ~3.5 keys.

Two sellers less than 5 keys, excluding 5 and anything over.

Going with 4 flat, supported by the untraceable bot.tf sales at ~3.5.

Closing due to new sales arising, and the likeliness of them affecting the price, especially given the new rule change. Posting the marketplace sale info below so I remember if I get to re-suggest this before someone else takes it.

As per the new rule regarding marketplace sales:

https://backpack.tf/item/6291884713

https://bot.tf/stats/listings/MVRRLXD6 - B/O for one of the sales in August was ~6.1 keys

https://backpack.tf/item/6868403047

https://bot.tf/stats/listings/MVRQ65EW

https://bot.tf/stats/listings/YDL15RLL - no listed B/O for this one

https://backpack.tf/item/6253390499

https://bot.tf/stats/listings/MVRQ65EW

https://bot.tf/stats/listings/YDL15RLL 0 no listed B/O for the other one either

https://backpack.tf/item/6366275253 - may be the history of the July sale, given the large gap between the bot's backpack scan:

https://backpack.tf/profiles/76561198135383628#!/compare/1563408000/1564444800

https://bot.tf/stats/listings/Y355ZV86 - no listed B/O