Using last month.

Sale 1:

Sold for 4 keys + 29 ref (~0.5 keys)

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1559779200/1559865600

Buyer: https://backpack.tf/profiles/76561198324290571#!/compare/1559174400/1559865600

https://backpack.tf/item/3609832464

https://i.imgur.com/qKU1oRT.png - bot.tf confirms

X = 4.5

Sale 2:

Sold to a quickbuyer for 3 keys + 30 ref (~0.5 keys)

Seller: https://backpack.tf/profiles/76561198850006472#!/compare/1559347200/1560643200

Buyer: https://backpack.tf/profiles/76561198097271466#!/compare/1560470400/1560643200

https://backpack.tf/item/7807804299

X = 3.5

Sale 3:

Sold for 3 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198334559541#!/compare/1561680000/1561766400

Buyer: https://backpack.tf/profiles/76561198447095964#!/compare/1561766400/1561852800

https://backpack.tf/item/7818562338

https://i.imgur.com/qKU1oRT.png - Labelled as 3 keys + 38 ref (~0.5 keys)

X = 3.5

Sale 4:

Sold for 3 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198841279602#!/compare/1561420800/1561507200

Buyer: https://backpack.tf/profiles/76561198066220918#!/compare/1551484800/1561507200

https://backpack.tf/item/7818562338

X = 3.5

Marketplace Sales:

https://marketplace.tf/items/tf2/30572;5;u3005 - The only two in the last month are ~$8.50, which comes to around 5 keys. This’ll be too high to use here.

Other Sales:

https://backpack.tf/item/7617494409 (#1)

https://backpack.tf/item/4391048754 (#2)

https://backpack.tf/item/4321040358 (#3)

https://backpack.tf/item/3630714829 (#4)

https://backpack.tf/item/7818562338 (#5 & #6)

https://backpack.tf/item/3841563614 (#7, the rest were gambled/scammed)

https://backpack.tf/item/3843056404 - traded between bot and bot owner

https://backpack.tf/item/3801886917 - traded between bot and bot owner?

https://backpack.tf/item/3847289860 - marketplace

https://backpack.tf/item/3626409228 - marketplace

https://backpack.tf/item/4445136576 - gambled

#1

Untraceable/bulk?

https://backpack.tf/profiles/76561198103502981#!/compare/1560211200/1561075200

https://backpack.tf/profiles/76561198060898226#!/compare/1560384000/1560729600

#2

Untraceable

https://backpack.tf/profiles/76561198333837293#!/compare/1560643200/1560729600

https://backpack.tf/profiles/76561198448830963#!/compare/1560729600/1560816000

https://i.imgur.com/qKU1oRT.png - labelled as 3 keys + 10 ref

#3

Bulk for Aces Das Ubersternmann

https://backpack.tf/profiles/76561198279609240#!/compare/1559606400/1559952000

https://backpack.tf/profiles/76561198120672630#!/compare/1559520000/1559606400

#4

Untraceable

https://backpack.tf/profiles/76561198311498888#!/compare/1560556800/1560643200

https://backpack.tf/profiles/76561198307156465#!/compare/1560643200/1560902400

#5

Untraceable, only 10 ref matches

https://backpack.tf/profiles/76561198286021479#!/compare/1560556800/1560643200

https://backpack.tf/profiles/76561198841279602#!/compare/1560556800/1560643200

#6

Untraceable

https://backpack.tf/profiles/76561198066220918#!/compare/1561507200/1561593600

https://backpack.tf/profiles/76561198334559541#!/compare/1561507200/1561593600

https://i.imgur.com/qKU1oRT.png - labelled as 3 keys + 21 ref

#7

Untraceable

https://backpack.tf/profiles/76561198334598690#!/compare/1561680000/1561766400

https://backpack.tf/profiles/76561198132111326#!/compare/1561680000/1561766400

https://i.imgur.com/qKU1oRT.png - labelled as 3 keys + 40 ref (~1)

Sellers:

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

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

https://backpack.tf/item/3633223335 - ~3.8 keys

https://backpack.tf/item/7807804299 - ~3.8 keys

https://backpack.tf/item/4391048754 - ~4.5 keys

https://backpack.tf/item/3843056404 - ~4.5 keys

https://backpack.tf/item/4332714640 - ~4.6 keys

https://backpack.tf/item/4321040358 - ~6.9 keys

https://backpack.tf/item/3606698788 - 18 keys

https://backpack.tf/item/5722206142 - 18 keys

https://backpack.tf/item/6684063426 - 18 keys

Sales at:

3.5 (3x)

4.5

4.5 is an outlier, excluding.

Going with 3.5 flat.