Using last month.

Sale 1:

Sold for 16 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524787200/1524873600

Buyer: https://backpack.tf/profiles/76561198005163008#!/compare/1524787200/1524873600

Untraceable, but bot.tf confirms: https://i.imgur.com/mb5a7fh.png

X = 17

Sale 2:

Sold for 16 keys

Seller: https://backpack.tf/profiles/76561198165677507#!/compare/1523923200/1524096000

Buyer: https://backpack.tf/profiles/76561198086152817#!/compare/1524009600/1524096000

X = 16

Sale 3:

Sold for $30.85 on marketplace.tf

https://marketplace.tf/items/1172;5;u3007

30.85/1.97 = ~15.66

X = 16

Sale 4:

Sold for 8 keys + ’72 Most Wanted (Mini at ~6.25 below)

http://www.tf2outpost.com/trade/29647589 - No B/O

Seller: https://backpack.tf/profiles/76561198100773215#!/compare/1525824000/1525910400

Buyer: https://backpack.tf/profiles/76561198046921199#!/compare/1525651200/1525910400

X = 14

Sale 5:

Bulk for Nebula Universal Translator

Seller: https://backpack.tf/profiles/76561198025934379#!/compare/1524528000/1524614400

Buyer: https://backpack.tf/profiles/76561198066195760#!/compare/1524528000/1524614400

’72 Most Wanted Mini:

https://marketplace.tf/items/30614;5;u3004

Sales ranging from ~6-7.5 keys worth

https://i.imgur.com/peWJWbG.png - Bot.tf sales around 5 keys

Taking 5-7.5 (~6.25)

Lazy-ass mini, but I’ll be getting around to doing the Most Wanted in like a week anyways.

Sellers:

16 keys:

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

https://backpack.tf/item/5643216156

35 keys:

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

https://backpack.tf/item/6134923859

50 keys:

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

https://backpack.tf/item/5443764157

Sales at:

14

16 (2x)

17

Sale at 17 is untraceable, excluding 17.

Going with 14-16.