Using last month.

Sale 1:

Sold for $18.99 on Marketplace.tf

https://marketplace.tf/items/30763;5;u3010

https://backpack.tf/item/6271209494

18.99/1.97 = ~9.64

X = 9.5

Sale 2:

Sold with a ref for 8 keys + Schadenfreude (~1)

Seller: https://backpack.tf/profiles/76561198124358410#!/compare/1523318400/1523491200

Buyer: https://backpack.tf/profiles/76561198124728984#!/compare/1523145600/1523404800

X = 9

Sale 3:

Sold for 8 keys + small amount of ref

Seller: https://backpack.tf/profiles/76561198046921199#!/compare/1525219200/1525305600

Buyer: https://backpack.tf/profiles/76561198153481237#!/compare/1525219200/1525305600

X = 8

Sale 4:

Bulk for Haunted Ghosts Head Hedge

Seller: https://backpack.tf/profiles/76561198051980490#!/compare/1523404800/1523491200

Buyer: https://backpack.tf/profiles/76561198243076082#!/compare/1523318400/1523491200

Sale 5:

Sold with ~4 keys in items for Ancient Codex Universal Translator

No outpost

Seller: https://backpack.tf/profiles/76561198394080554#!/compare/1522972800/1523059200

Buyer: https://backpack.tf/profiles/76561198317156035#!/compare/1522454400/1523059200

Looks like a bit of an outlier. Excluding.

Sale 6:

Sold for SGG Pool Party

http://www.tf2outpost.com/trade/29616441 - B/O of 15

Seller: https://backpack.tf/profiles/76561198124728984#!/compare/1523577600/1523750400

Buyer: https://backpack.tf/profiles/76561198116732510#!/compare/1523664000/1523750400

Pool is in-date at 17, but has sellers at ~12.5. Might be better to just exclude this sale entirely.

Sellers:

~15 keys:

https://i.imgur.com/lP6qcP9.png

https://backpack.tf/item/6665896856

https://backpack.tf/item/5438243227

17 keys:

https://i.imgur.com/nEFQx0R.png

https://backpack.tf/item/5014073206

https://backpack.tf/item/5380120298

https://backpack.tf/item/6483195382

https://backpack.tf/item/5118425410

20 keys:

https://i.imgur.com/9wBVblV.png

https://backpack.tf/item/4964904060

40 keys:

https://i.imgur.com/35WD8Nu.png

https://backpack.tf/item/4962067775

And two sellers on the SCM for ~8.5-9 keys worth:

https://i.imgur.com/Qp1Fetk.png

Sales at:

8

9

9.5

Based on the sellers, the two sales that were excluded would end up being close to ~12 keys each. Given the two sellers on the SCM, these will end up being outliers.

Going with 8-9.5.