All sales are from the last 30 days

Sale 1:

Sold for Australium Scattergun (~27)

Seller: https://backpack.tf/profiles/76561198088775634#!/compare/1514592000/1514678400

Buyer: https://backpack.tf/profiles/76561198065643740#!/compare/1514332800/1515024000

X = 27 :'(

Sale 2:

Sold for Green Confetti Birdcage (Mini at 11 below)

http://www.tf2outpost.com/trade/29527261 - B/O of 11

Seller: https://backpack.tf/profiles/76561198190498499#!/compare/1516406400/1516492800

Buyer: https://backpack.tf/profiles/76561198102420057#!/compare/1516406400/1516492800

X = 11

Sale 3:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198063640894#!/compare/1516579200/1516665600

Buyer: https://backpack.tf/profiles/76561198196939174#!/compare/1516579200/1516752000

X = 10

Sale 4:

Sold for Mega Strike Dueling Banjo

Seller: https://backpack.tf/profiles/76561198102420057#!/compare/1516492800/1516579200

Buyer: https://backpack.tf/profiles/76561198139229580#!/compare/1516492800/1516579200

Probably use on the Banjo

Green Confetti Birdcage Mini:

Sold for 11 keys + 4 ref

Seller: https://backpack.tf/profiles/76561198066195760#!/compare/1515456000/1515542400

Buyer: https://backpack.tf/profiles/76561198186895735#!/compare/1515542400/1515715200

X = 11

Sellers:

~10.3 keys:

https://i.imgur.com/uAqxk1J.png

11 keys:

https://i.imgur.com/JGebwdK.png - 1 day

15 keys:

https://i.imgur.com/Ot1bp53.png - 16 days

19 keys:

http://www.tf2outpost.com/trade/28708448 - 1 year, probably lowered at some point

20 keys:

https://i.imgur.com/SFW1J9f.png - 5 months

https://i.imgur.com/Pgie4ZG.png - 1 month

http://www.tf2outpost.com/trade/29494287 - 3 weeks

35 keys:

https://i.imgur.com/6Hl1PmI.png - 1 month

Offers:

http://www.tf2outpost.com/trade/29506332 - 2 weeks

Sales at:

10

11

27

27 is a large outlier, excluding 27.

Going with 10-11.