Using last month

Sale 1:

Sold for 16 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523750400/1524096000

Buyer: https://backpack.tf/profiles/76561198427925598#!/compare/1524009600/1524096000

Bot.tf confirms: https://i.imgur.com/UdteI46.png

X = 16

Sale 2:

Another bot.tf sale at 16 keys

https://i.imgur.com/UdteI46.png

Can’t find the sale

X = 16

Sale 3:

Sold for Peace Sign Ze Goggles (Mini at 14 below)

http://www.tf2outpost.com/trade/29604474 - B/O of 16

Seller: https://backpack.tf/profiles/76561198055622215#!/compare/1524182400/1524268800

Buyer: https://backpack.tf/profiles/76561198394903250#!/compare/1524182400/1524268800

X = 14

Peace Sign Goggles Mini:

Sold for 13 keys + ~1 key in metal

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1518220800/1518307200

Buyer: https://backpack.tf/profiles/76561198353902516#!/compare/1518220800/1518307200

X = 14

Sellers:

20 keys:

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

https://backpack.tf/item/3424653197

27 keys:

https://i.imgur.com/JopBQ4p.png

https://backpack.tf/item/2971943376

30 keys:

https://i.imgur.com/xnLN7I4.png

https://backpack.tf/item/2694855684

SCM: (taking keys as $2.47)

https://i.imgur.com/faPJc8Q.png (only the first 2 are important)

33.64 (~13.6 keys)

36.43 (~14.7 keys)

Sales at:

14

16 (2x)

Going with 14-16.