Sale 1:

Sold for 6 keys (ID's match)

Seller: https://backpack.tf/profiles/76561198069441398#!/compare/1522713600/1522972800

Buyer: https://backpack.tf/profiles/76561198081073113#!/compare/1522800000/1523059200

X = 6

Sale 2:

Sold for $12 on Marketplace.tf

https://marketplace.tf/items/30616;5;u3008

Can't find item's history

12/1.98 = ~6.06

X = 6

Sale 3:

Sold for 5 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198301897810#!/compare/1522022400/1522108800

Buyer: https://backpack.tf/profiles/76561198073830880#!/compare/1521590400/1522108800

X = 5.5

Sale 4:

Quicksold for 5 keys

Seller: https://backpack.tf/profiles/76561198097135729#!/compare/1522454400/1522540800

Buyer: https://backpack.tf/profiles/76561198146326272#!/compare/1522454400/1522540800

X = 5

Sale 5:

Quicksold for 5 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198146326272#!/compare/1522454400/1522540800

Buyer: https://backpack.tf/profiles/76561198067306890#!/compare/1522368000/1522540800

X = 5

Sale 6:

Quicksold for 4 keys + 5 ref

Seller: https://backpack.tf/profiles/76561198090457223#!/compare/1523923200/1524096000

Buyer: https://backpack.tf/profiles/76561198258264645#!/compare/1524009600/1524096000

X = 4

Sellers:

~6.73 keys:

https://i.imgur.com/McR144B.png

https://backpack.tf/item/4339916767

https://backpack.tf/item/5762375352

8 keys:

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

https://backpack.tf/item/4455963936

http://www.tf2outpost.com/trade/29617066

12 keys:

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

https://backpack.tf/item/6536614994

16 keys:

https://i.imgur.com/VTQ4cTS.png

https://backpack.tf/item/6400394795

Sales at:

4

5

5.5

6

Sales at 4 and 5 quicksold, excluding.

Going with 5.5-6.