Using last month.

Sales 1 & 2:

Sold for $22.99 and $8.05 on marketplace.tf

https://marketplace.tf/items/30572;5;u3004

22.99/1.94 = ~11.9

8.05/1.98 = ~4.07

X = 12

X = 4

Sale 3:

Sold for 8 keys + 6 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1525737600/1525824000

Buyer: https://backpack.tf/profiles/76561197963394917#!/compare/1525737600/1525824000

Bot.tf confirms: https://i.imgur.com/xXMOzKi.png (ignoring the buy because it came from a scammer)

X = 8

Sale 4:

Sold for 7 keys + Dead of Night (~0.5)

Seller: https://backpack.tf/profiles/76561198172458063#!/compare/1527292800/1527379200

Buyer: https://backpack.tf/profiles/76561197992998498#!/compare/1526428800/1527379200

X = 7.5

Sale 5:

Sold for Nuts n’ Bolts Your Worst Nightmare

http://www.tf2outpost.com/trade/29657137 - B/O of 13

Seller: https://backpack.tf/profiles/76561198162548251#!/compare/1526601600/1526688000

Buyer: https://backpack.tf/profiles/76561198131895768#!/compare/1526601600/1526688000

No mini available on the nightmare. Given the 2 month old seller at 13, I doubt it would retain its value of 13 anyways, and would most likely end up falling into the range. Supports the high-end, too.

Sale 6:

Bulk for Disco Duck

Seller: https://backpack.tf/profiles/76561198064104470#!/compare/1527379200/1527465600

Buyer: https://backpack.tf/profiles/76561198074924284#!/compare/1527379200/1527465600

Sellers:

12 keys:

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

https://backpack.tf/item/4546393821

https://backpack.tf/item/3867050260

16 keys:

https://i.imgur.com/8E4dIH3.png

https://backpack.tf/item/3596650066

20 keys:

https://i.imgur.com/YB3WNmm.png

https://backpack.tf/item/3769342125

https://backpack.tf/item/6599252912

Sales at:

4

7.5

8

12

4 is an outlier, excluding 4.

Going with 7.5-12 (probably the worst looking range so far).