Using last 2 months.

Sale 1:

Sold for Memory Leak Bonk Leadwear (Suggestion at 13)

No outpost

Seller: https://backpack.tf/profiles/76561198314352326#!/compare/1522195200/1522281600

Buyer: https://backpack.tf/profiles/76561198116483973#!/compare/1522195200/1522281600

X = 13

Sale 2:

Sold for 8 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198386441509#!/compare/1521676800/1521763200

Buyer: https://backpack.tf/profiles/76561198077346034#!/compare/1521763200/1521849600

Bot.tf confirms: https://i.imgur.com/eZp5kRS.png

X = 8.5

Sale 3:

Sold for Silver Cyclone Balloonibouncer (6.5) + 1 key

No outpost

Seller: https://backpack.tf/profiles/76561198043936328#!/compare/1522540800/1522627200

Buyer: https://backpack.tf/profiles/76561198005121419#!/compare/1522540800/1522627200

X = 7.5

Sale 4:

Sold for 7 keys

https://i.imgur.com/763gW50.png

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1524873600/1524960000

Buyer: https://backpack.tf/profiles/76561198081081818#!/compare/1524873600/1524960000

X = 7

Sale 5:

Bulk for Peace Noble Nickel

Seller: https://backpack.tf/profiles/76561198259098579#!/compare/1523145600/1523318400

Buyer: https://backpack.tf/profiles/76561198259369182#!/compare/1523145600/1523318400

Sale 6:

Bulk for Burning Defiant Spartan

Seller: https://backpack.tf/profiles/76561198085984093#!/compare/1522108800/1522454400

Buyer: https://backpack.tf/profiles/76561198043936328#!/compare/1522368000/1522454400

Memory Leak Bonk Leadwear:

https://backpack.tf/suggestion/5ae5641944325a3b36237944

Sellers:

13 keys:

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

https://backpack.tf/item/6643740051

30 keys:

https://i.imgur.com/yWLeM4S.png

https://backpack.tf/item/6393398983

Sales at:

7

7.5

8.5

13

13 is an outlier, excluding 13.

Going with 7-8.5.