Using sales from this month.

Sale 1:

Sold for 5 keys + ~0.5 keys in metal/items

Seller: https://backpack.tf/profiles/76561198382589541#!/compare/1531872000/1531958400

Buyer: https://backpack.tf/profiles/76561198157171180#!/compare/1531872000/1531958400

https://i.imgur.com/X0Vs3Rd.png - B/O of 5.5, according to bot.tf

X = 5.5

Sale 2:

Sold for 5 keys + 18 ref

Seller: https://backpack.tf/profiles/76561198157171180#!/compare/1532649600/1532736000

Buyer: https://backpack.tf/profiles/76561198122916546#!/compare/1532476800/1532736000

X = 5.5

Sale 3:

Sold with ~0.5 keys in metal for 6 keys

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1531958400/1532044800

Buyer: https://backpack.tf/profiles/76561198365931332#!/compare/1531958400/1532044800

Bot.tf confirms: https://i.imgur.com/X0Vs3Rd.png

X = 5.5

Sale 4:

Sold with a small amount of metal for 5 keys (Buyer’s compare link)

Seller: https://backpack.tf/profiles/76561198076610032#!/compare/1530576000/1530835200

Buyer: https://backpack.tf/profiles/76561197961066474#!/compare/1530835200/1530921600

X = 5

Sale 5:

Sold to a bot for 5 keys + 4 ref?

Seller: https://backpack.tf/profiles/76561198165205268#!/compare/1530403200/1531008000

Buyer: https://backpack.tf/profiles/76561198382589541#!/compare/1530316800/1530403200

Untraceable, apart from 3 ref. Bot.tf confirms, but it is unreliable. https://i.imgur.com/X0Vs3Rd.png

X = 5?

Sale 6:

Sold to a bot for 4 keys + 4 ref?

Seller: https://backpack.tf/profiles/76561198211445756#!/compare/1531958400/1532044800

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1531958400/1532044800

Only 4 keys are traceable, though I doubt bot.tf is incorrect here. https://i.imgur.com/X0Vs3Rd.png

X = 4

Sale 7:

Sold to a bot for 3 keys + 8 ref

Seller: https://backpack.tf/profiles/76561198242607954#!/compare/1532217600/1532304000

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1532131200/1532217600

Bot.tf confirms: https://i.imgur.com/X0Vs3Rd.png

X = 3

Sellers:

~5.6 keys:

https://i.imgur.com/89NWEsT.png

https://backpack.tf/item/4257901012

~5.8 keys:

https://i.imgur.com/2KZVbZG.png

https://backpack.tf/item/4018951572

7 keys:

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

https://backpack.tf/item/5522664520

12 keys:

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

https://backpack.tf/item/3962786787

14 keys:

https://i.imgur.com/SvhaJ3r.png

https://backpack.tf/item/2724946277

https://backpack.tf/item/2898249065

16 keys:

https://i.imgur.com/vd4DN60.png

https://backpack.tf/item/2898249065

Offers:

http://www.tf2outpost.com/trade/29688661

Sales at:

3

4

5

5.5 (3x)

And an untraceable sale at 5.

3 is way too low, excluding.

Sale at 4 went to a bot, and is not a common trading point. Excluding 4.

Going with 5-5.5.