Premium ran out while making this :(

Sales 1 & 2:

Sold for $52 and $33.64 on Marketplace.tf

https://marketplace.tf/items/1172;5;u3009

Can't find item's history

52/1.98 = 26.26

33.64/1.98 = ~16.99

X = 26

X = 17

Sale 3:

Sold for 11 keys (Buyer's compare link)

Seller: https://backpack.tf/profiles/76561198176382914#!/compare/1521936000/1522454400

Buyer: https://backpack.tf/profiles/76561198102964652#!/compare/1522368000/1522454400

X = 11

Sellers:

15 keys:

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

https://backpack.tf/item/5405571233

16 keys:

https://i.imgur.com/qHMWC2h.png

https://backpack.tf/item/5196365346 (has it listed for ~15 keys worth on marketplace.tf)

https://i.imgur.com/bt0qT5d.png

https://backpack.tf/item/6699031975

24 keys:

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

https://backpack.tf/item/6004192459

Marketplace.tf:

https://marketplace.tf/items/1172;5;u3009

Only noteworthy one that hasn't been pointed out yet is the one for $30, which is roughly 15 keys.

SCM:

https://i.imgur.com/44MA3gE.png

$37 (~14.98 keys)

$37.30 (~15.1 keys)

$38.03 (~15.4 keys)

$39.99 (~16.19 keys)

$41.95 (~16.98 keys)

$43.18 (~17.48 keys)

Others aren't too important here.

Sales at:

11

17

26

Several buyers significantly higher than 11:

https://i.imgur.com/qB0f0wX.png

Excluding 11.

26 is an outlier, excluding 26.

Sellers at ~15 keys. Using that as the low-end.

Going with 15-17.