Using last month.

Sales 1 & 2:

Sold for $19 and $14.89 on Marketplace.tf

https://marketplace.tf/items/30673;5;u3010

19/1.97 = ~9.64

14.89/1.97 = ~7.56

X = 9.5

X = 7.5

Sale 3:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198086076383#!/compare/1524096000/1524182400

Buyer: https://backpack.tf/profiles/76561198044706910#!/compare/1524096000/1524182400

X = 8

Sale 4:

Sold for 3 keys + ~4 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198353268162#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198122572748#!/compare/1523145600/1523232000

X = 7

Sale 5:

Sold for 5 keys, most likely to a quickbuyer

Seller: https://backpack.tf/profiles/76561198412089276#!/compare/1524441600/1524528000

Buyer: https://backpack.tf/profiles/76561198067306890#!/compare/1524441600/1524528000

X = 5

Sale 6:

Quicksold for 2 keys + ~1 key in ref + ~2 keys in items

Seller: https://backpack.tf/profiles/76561198268615482#!/compare/1525305600/1525392000

Buyer: https://backpack.tf/profiles/76561198349635856#!/compare/1525305600/1525392000

X = 5

Sale 7:

Sold for 3 keys + cheap items

Seller: https://backpack.tf/profiles/76561198268676455#!/compare/1525305600/1525392000

Buyer: https://backpack.tf/profiles/76561198268615482#!/compare/1525305600/1525392000

X = 3

Sale 8:

Sold for Grail Breakdance

No outpost

Seller: https://backpack.tf/profiles/76561198349635856#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198273294595#!/compare/1525392000/1525478400

Sale 9:

Sold with a key for some unusual smg

Seller: https://backpack.tf/profiles/76561198157804786#!/compare/1524268800/1524355200

Buyer: https://backpack.tf/profiles/76561198138276570#!/compare/1524268800/1524355200

Most likely too low on the SMG, and too high here.

Sale 10:

Sold with a key for Miami Nights Bolted Birdcage

No outpost

Seller: https://backpack.tf/profiles/76561198214618631#!/compare/1525046400/1525132800

Buyer: https://backpack.tf/profiles/76561198254039787#!/compare/1525046400/1525132800

Seems kinda high, excluding.

Sellers:

8 keys:

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

https://backpack.tf/item/4529170914

14 keys:

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

https://backpack.tf/item/4981844474

20 keys:

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

https://backpack.tf/item/6086477286

25 keys:

http://www.tf2outpost.com/trade/29627985

30 keys:

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

https://backpack.tf/item/4256876151

http://www.tf2outpost.com/trade/29505585

http://www.tf2outpost.com/trade/29597222

999 keys:

https://i.imgur.com/rps1ec5.png

https://backpack.tf/item/4691711859

Offers:

http://www.tf2outpost.com/trade/29585654

Sales at:

3

5 (2x)

7

7.5

8

9.5

The sales at 5 looks to have gone to quickbuyers, or just quicksold. Excluding 3 and 5.

9.5 looks too high with the seller at 8 on the classifieds. Excluding 9.5

Going with 7-8.