Sale 1:

Sold for 25 keys

Seller: https://backpack.tf/profiles/76561198055068182#!/compare/1523145600/1523318400

Buyer: https://backpack.tf/profiles/76561198109725712#!/compare/1522972800/1523404800

X = 25

Sale 2:

Sold for Grail Battin' a Thousand (Mini at 6 below) + ~2.5 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198063640894#!/compare/1524096000/1524182400

Buyer: https://backpack.tf/profiles/76561198812788964#!/compare/1524096000/1524182400

X = 8.5

Sale 3:

Sold for Whirlwind Second Rate Sorcery (8.25)

Seller: https://backpack.tf/profiles/76561198141152637#!/compare/1524268800/1524355200

Buyer: https://backpack.tf/profiles/76561198128012200#!/compare/1524268800/1524441600

X = 8.5

Sale 4:

Sold with ~2.5 keys in pure/items for Strange Pro. KS Knife (~9)

https://i.imgur.com/TPlbcdx.png - B/O of 9

Seller: https://backpack.tf/profiles/76561198812788964#!/compare/1524441600/1524614400

Buyer: https://backpack.tf/profiles/76561198282962719#!/compare/1524441600/1524614400

X = 7.5

Sale 5:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198121830870#!/compare/1523491200/1523577600

Buyer: https://backpack.tf/profiles/76561198041723076#!/compare/1522886400/1523577600

X = 6

Holy Grail Battin' Mini:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1522713600/1522800000

Buyer: https://backpack.tf/profiles/76561198097271466#!/compare/1522713600/1522800000

X = 6

(Will do a full suggestion once I get around to it)

Sellers:

8 keys:

https://i.imgur.com/v37VVbd.png

https://backpack.tf/item/6643000743

22 keys:

http://www.tf2outpost.com/trade/29597608

SCM supports:

https://steamcommunity.com/market/listings/440/Unusual%20Taunt%3A%20The%20Killer%20Solo?filter=silver

Sales at:

6

7.5

8.5 (2x)

25

25 is an outlier, excluding.

SCM buyers at ~6 keys worth, excluding 6.

Going with 7.5-8.5.