Using last month.

Sales 1 & 2:

Sold for $19.80 and $16.99 on Marketplace.tf

https://marketplace.tf/items/1106;5;u3008

19.8/1.95 = ~10.2

16.99/1.96 = ~8.67

X = 10

X = 8.5

Sale 3:

Sold for 8 keys + 13 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524960000/1525046400

Buyer: https://backpack.tf/profiles/76561198039810851#!/compare/1524873600/1525046400

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

Buyer’s end shows a Miami nights Gatsby was removed. It was sold to a quickbuyer, and then the buyer used the keys to buy the taunt. Seller’s end shows only 4 keys + 13 ref. All key histories can be found at the bottom here:

https://backpack.tf/profiles/76561198024925599#!/compare/1524960000/1525046400 (Miami Gatsby buyer)

X = 8.5

Sellers:

9 keys:

https://i.imgur.com/t2q7vUq.png

https://backpack.tf/item/2886454467

https://backpack.tf/item/2724673451

14 keys:

https://i.imgur.com/jKuVm2Z.png

https://backpack.tf/item/3379341868

17 keys:

https://i.imgur.com/78Gp8eO.png

https://backpack.tf/item/3124118830

35 keys:

https://i.imgur.com/6WRYymQ.png

https://backpack.tf/item/2697999531

Sales at:

8.5 (2x)

10

10 looks like an outlier, and sellers at 9 are several weeks old. Excluding 10.

Going with 8.5.