Using last month.

Sale 1:

Sold for 4 lime paints (~9 total at the time) + 2 keys + some ref

http://www.tf2outpost.com/trade/29611278 - No visible B/O

Seller: https://backpack.tf/profiles/76561198086076383#!/compare/1522972800/1523059200

Buyer: https://backpack.tf/profiles/76561198053186338#!/compare/1522972800/1523059200

X = 11

Sale 2:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198097271466#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198047597929#!/compare/1525305600/1525392000

Key histories, in case the seller's link changes later today:

https://backpack.tf/item/6258255904

https://backpack.tf/item/5920648082

https://backpack.tf/item/4121469308

https://backpack.tf/item/2606744590

https://backpack.tf/item/5419814891

https://backpack.tf/item/4935183250

https://backpack.tf/item/5723523783

https://backpack.tf/item/3611893309

Key histories came from guns, looks like guns bought the aus flamethrower for 10 keys, and sold it to a bot immediately

https://backpack.tf/profiles/76561198000889832#!/compare/1525219200/1525392000

X = 8

Sale 3:

Bulk for Cloud 9 Tundra Top

Seller: https://backpack.tf/profiles/76561198069910864#!/compare/1523664000/1524096000

Buyer: https://backpack.tf/profiles/76561198062840129#!/compare/1523923200/1524096000

Sellers:

8 keys:

https://i.imgur.com/N3rgBzH.png

https://backpack.tf/item/3977181220

~8.9 keys:

https://i.imgur.com/4ugn7kL.png

https://backpack.tf/item/5739210696

Offers:

http://www.tf2outpost.com/trade/29623523

Sales at:

8

11

11 looks like a large outlier, given the cheaper sellers, and the month-unsold at ~8.9 keys. It's also almost a month old now. Excluding 11.

Going with 8 flat.