Using last month.

Marketplace Sales:

https://marketplace.tf/items/tf2/1117;5;u3003

$7.83 on Jul 18th (2x)

$6.10 on Jul 14th (4x)

$6.10 on Jul 12th

$4.87 on Jul 11th

$7.83 on Jul 8th

https://manic.tf/keyprice/?at=2019-07-08 - $7.83 / $1.83 = 4.28 keys

https://manic.tf/keyprice/?at=2019-07-11 - $4.87 / $1.85 = 2.63 keys

https://manic.tf/keyprice/?at=2019-07-12 - $6.10 / $1.83 = 3.33 keys

https://manic.tf/keyprice/?at=2019-07-14 - $6.1 / $1.82 = 3.35 keys (2x)

https://manic.tf/keyprice/?at=2019-07-18 - $7.83 / $1.82 = 4.30 keys (4x)

X = 4.5 (5x)

X = 3.5 (3x)

X = 2.5

Other Sales;

https://backpack.tf/item/4973811200 (#1 & #2)

https://backpack.tf/item/4893242099 (#3 & #4)

https://backpack.tf/item/3755951630 (#5)

https://backpack.tf/item/6353932389 (#6-8)

https://backpack.tf/item/2789758953 (#9 & #10)

https://backpack.tf/item/2748808353 (#11 & #12)

https://backpack.tf/item/2720555070 (#13)

https://backpack.tf/item/2711399001 (#14-18)

https://backpack.tf/item/2692839555 (#19-21)

https://backpack.tf/item/6483942841 - gambled/untraceable

https://backpack.tf/item/5916372940 - tradeit

https://backpack.tf/item/3220865002 - scrap/tradeit/gambled/untraceable

https://backpack.tf/item/3107385549 - marketplace

#1

Untraceable

https://backpack.tf/profiles/76561198334598690#!/compare/1562803200/1562889600

https://backpack.tf/profiles/76561198252661528#!/compare/1559692800/1562976000

https://i.imgur.com/v9mzg6W.png - labelled as a 3 key + 20 ref sale (~3.5 keys). The purchase is also untraceable, labelled as a 2 key + 36 ref purchase (~2.5 keys)

#2

Untraceable

https://backpack.tf/profiles/76561198340634356#!/compare/1562544000/1562630400

https://backpack.tf/profiles/76561198063075911#!/compare/1562544000/1562630400

https://i.imgur.com/v9mzg6W.png - labelled as a 3 key + 20 ref sale (~3.5 keys)

#3

Untraceable

https://backpack.tf/profiles/76561198119531598#!/compare/1563148800/1563235200

https://backpack.tf/profiles/76561198121830870#!/compare/1563321600/1563408000

The 2nd link here probably wasn’t the buyer.

#4

Untraceable

https://backpack.tf/profiles/76561198121830870#!/compare/1563321600/1563408000

https://backpack.tf/profiles/76561198097171432#!/compare/1562889600/1563408000

I’m confident that it sold for the 3 keys in the seller’s end, and went to a different buyer.

The item was gambled after this.

#5

Untraceable

https://backpack.tf/profiles/76561198119967860#!/compare/1563926400/1564358400

https://backpack.tf/profiles/76561198340634356#!/compare/1564272000/1564358400

https://i.imgur.com/v9mzg6W.png - labelled as a 2 key + 36 ref purchase (~2.5 keys)

#6

Untraceable

https://backpack.tf/profiles/76561198078373457#!/compare/1563580800/1563667200

https://backpack.tf/profiles/76561198340634356#!/compare/1563580800/1563667200

https://i.imgur.com/v9mzg6W.png - labelled as a 2 key + 36 ref purchase (~2.5 keys)

The sale after this is also untraceable

#7

Untraceable

https://backpack.tf/profiles/76561198164353433#!/compare/1563926400/1564099200

https://backpack.tf/profiles/76561198340634356#!/compare/1564012800/1564099200

https://i.imgur.com/v9mzg6W.png - labelled as a 2 key + 36 ref purchase (~2.5 keys)

#8

Untraceable

https://backpack.tf/profiles/76561198340634356#!/compare/1564099200/1564185600

https://backpack.tf/profiles/76561198101300189#!/compare/1564099200/1564185600

https://i.imgur.com/v9mzg6W.png - labelled as a 3 key + 20 ref sale (~3.5 keys)

Gambled afterwards.

#9

Untraceable

https://backpack.tf/profiles/76561198164353433#!/compare/1562976000/1563062400

https://backpack.tf/profiles/76561198363395191#!/compare/1563235200/1563321600

#10

SCM

https://backpack.tf/profiles/76561198363395191#!/compare/1563321600/1563408000

https://backpack.tf/profiles/76561198241780530#!/compare/1563321600/1563753600

#11

Untraceable

https://backpack.tf/profiles/76561198965695956#!/compare/1562198400/1562803200

https://backpack.tf/profiles/76561198340634356#!/compare/1562716800/1562803200

https://i.imgur.com/v9mzg6W.png - labelled as a 2 key + 36 ref purchase (~2.5 keys)

#12

Untraceable, only 1 key and some ref matches

https://backpack.tf/profiles/76561198340634356#!/compare/1563494400/1563580800

https://backpack.tf/profiles/76561198208221697#!/compare/1563580800/1563926400

#13

Untraceable

https://backpack.tf/profiles/76561198271892201#!/compare/1562976000/1563062400

https://backpack.tf/profiles/76561198078219409#!/compare/1561852800/1563148800

#14

SCM

https://backpack.tf/profiles/76561198031007256#!/compare/1562889600/1562976000

https://backpack.tf/profiles/76561198273183085#!/compare/1562803200/1563148800

#15

Untraceable

https://backpack.tf/profiles/76561198273183085#!/compare/1563408000/1563667200

https://backpack.tf/profiles/76561198200908353#!/compare/1563580800/1563667200

#16

Untraceable

https://backpack.tf/profiles/76561198200908353#!/compare/1563580800/1563667200

https://backpack.tf/profiles/76561198340634356#!/compare/1563580800/1563667200

https://i.imgur.com/v9mzg6W.png - labelled as a 2 key + 36 ref purchase (~2.5 keys)

#17

Untraceable

https://backpack.tf/profiles/76561198340634356#!/compare/1563926400/1564012800

https://backpack.tf/profiles/76561198069472823#!/compare/1563926400/1564012800

https://i.imgur.com/v9mzg6W.png - labelled as a 3 key + 20 ref sale (~3.5 keys)

#18

Untraceable

https://backpack.tf/profiles/76561198069472823#!/compare/1563926400/1564012800

https://backpack.tf/profiles/76561198412245151#!/compare/1560988800/1564012800

#19

Untraceable

https://backpack.tf/profiles/76561198340634356#!/compare/1562544000/1562630400

https://backpack.tf/profiles/76561198063075911#!/compare/1562544000/1562630400

https://i.imgur.com/v9mzg6W.png - labelled as a 3 key + 20 ref sale (~3.5 keys)

Gambled afterwards

#20

Untraceable

https://backpack.tf/profiles/76561198120672630#!/compare/1563062400/1563148800

https://backpack.tf/profiles/76561198363395191#!/compare/1563235200/1563321600

#21

Untraceable, only 1 key + 11 ref matches

https://backpack.tf/profiles/76561198363395191#!/compare/1563321600/1563408000

https://backpack.tf/profiles/76561198855132619#!/compare/1563321600/1563408000

Sellers:

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

https://i.imgur.com/c8CGWL8.png

https://backpack.tf/item/3755951630 - ~3.4 keys

https://backpack.tf/item/2711893977 - ~3.9 keys

https://backpack.tf/item/6353932389 - 4 keys

https://backpack.tf/item/6764364653 - 4 keys

https://backpack.tf/item/2733085617 - ~4 keys

https://backpack.tf/item/3544726075 - ~4.4 keys

https://backpack.tf/item/3391708776 - ~5.9 keys

https://backpack.tf/item/3935312248 - 6 keys

https://backpack.tf/item/2767704742 - 6 keys

https://backpack.tf/item/4897732502 - 10 keys

Sales at:

2.5

3.5 (3x)

4.5 (5x)

Basically just going to go with what bot.tf sales are at, which is 2.5 and 3.5.

Going with 2.5-3.5.