Using last month.

Sales 1 & 2:

Sold for $18.77 and $15 on Marketplace.tf

https://marketplace.tf/items/1116;5;u3007

18.77/1.97 = ~9.53

15/1.97 = ~7.61

X = 9.5

X = 7.5

Sale 3:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523923200/1524009600

Buyer: https://backpack.tf/profiles/76561198423070484#!/compare/1523750400/1524096000

Untraceable, but bot.tf confirms: https://i.imgur.com/s4QctPY.png

X = 6

Sale 4:

Sold for 5 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1525910400/1525996800

Buyer: https://backpack.tf/profiles/76561198825250125#!/compare/1525910400/1525996800

Bot.tf confirms: https://i.imgur.com/cjO28HA.png (credit to woifi for the screenshot)

X = 5.5

Sale 5:

Sold for 4 keys + 25 ref (Buyer’s compare link)

Seller: https://backpack.tf/profiles/76561198357046243#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198809718054#!/compare/1523836800/1523923200

Bot.tf confirms: https://i.imgur.com/s4QctPY.png

X = 5

Sale 6:

Sold for 4 keys + 28 ref

Seller: https://backpack.tf/profiles/76561198442764620#!/compare/1523923200/1524009600

Buyer: https://backpack.tf/profiles/76561198010245943#!/compare/1520812800/1524009600

Untraceable, but bot.tf confirms: https://i.imgur.com/s4QctPY.png

X = 5

Sale 7:

Sold in an auction for 5 keys?

Seller: https://backpack.tf/profiles/76561198084854664#!/compare/1525564800/1525996800

Buyer: https://backpack.tf/profiles/76561198101538371#!/compare/1525824000/1525996800

X = 5?

Sale 8:

Sold for 4 keys + 20 ref

No outpost

Seller: https://backpack.tf/profiles/76561198352286528#!/compare/1524009600/1524441600

Buyer: https://backpack.tf/profiles/76561198045466865#!/compare/1524355200/1524528000

X = 4.5

Sellers:

~5.5 keys:

https://i.imgur.com/CkAu5z3.png

https://backpack.tf/item/3529210627

https://backpack.tf/item/6408716772

Sales at:

4.5

5 (3x)

5.5

6

7.5

9.5

Going with 4.5-5.5, as suggested by Woifi.