Sale 1:

Sold for 13 keys

http://www.tf2outpost.com/trade/29291232 - B/O of 13

Seller: https://backpack.tf/profiles/76561198045692488#!/compare/1502409600/1502496000

Buyer: https://backpack.tf/profiles/76561198170060345#!/compare/1502409600/1502496000

X = 13

Sale 2:

Sold for 12 keys (IDs match)

http://www.tf2outpost.com/trade/29364476 - B/O of 12

Seller: https://backpack.tf/profiles/76561198045692488#!/compare/1505260800/1505347200

Buyer: https://backpack.tf/profiles/76561198286047775#!/compare/1505088000/1505347200

X = 12

Sale 3:

Sold for 11 keys + 15 ref

No outpost

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1506470400/1506556800

Buyer: https://backpack.tf/profiles/76561198230109280#!/compare/1506470400/1506556800

X = 11

Sale 4:

Sold for 9 keys

http://www.tf2outpost.com/trade/29385007 - B/O of 10

Seller: https://backpack.tf/profiles/76561198067235846#!/compare/1506470400/1506816000

Buyer: https://backpack.tf/profiles/76561198000091310#!/compare/1506729600/1506816000

X = 9

Sellers:

~11.4 keys:

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

13 keys:

https://i.imgur.com/OSJdgz7.png - 19 days

Sales at:

9

11

12

13

Buyers for any effect at 9 keys, and a buyer for steaming at 10:

https://i.imgur.com/TY0zkQP.png

Excluding 9.

Going with 11-13.