Sale 1:

Sold with ~5 keys worth for Terror-Watt Harmburg (Suggestion at 22) + Pro. KS Aussie Frontier Justice (12)

No outpost

Seller: https://backpack.tf/profiles/76561198118254940#!/compare/1512950400/1513036800

Buyer: https://backpack.tf/profiles/76561198013063543#!/compare/1512950400/1513036800

X = 29

Sale 2:

Sold for Blizzardy Storm Backwards Ballcap (Suggestion at 22.5) + 5 keys

http://www.tf2outpost.com/trade/29478632 - B/O of 30

Seller: https://backpack.tf/profiles/76561198095022965#!/compare/1513641600/1513728000

Buyer: https://backpack.tf/profiles/76561198072624990#!/compare/1513641600/1513728000

Rounding down, the high-end on the Ballcap is ~24.5, which would make this 22.25, which rounds to 22.

X = 27

Sale 3:

Sold for 24 keys

Seller: https://backpack.tf/profiles/76561198081129776#!/compare/1515024000/1515110400

Buyer: https://backpack.tf/profiles/76561198098812857#!/compare/1514937600/1515196800

X = 24

Sale 4:

Sold with 3 keys for Blizzardy Storm Trophy Belt (24.5)

No outpost

Seller: https://backpack.tf/profiles/76561198072624990#!/compare/1513814400/1513900800

Buyer: https://backpack.tf/profiles/76561198205424456#!/compare/1513900800/1513987200

X = 22

Sale 5:

Bulk for Slumber HWNN

Seller: https://backpack.tf/profiles/76561198013063543#!/compare/1513036800/1513123200

Buyer: https://backpack.tf/profiles/76561198095022965#!/compare/1512950400/1513036800

Sale 6:

Bulk for Anti-Freeze Attendant

Seller: https://backpack.tf/profiles/76561198205424456#!/compare/1513987200/1514073600

Buyer: https://backpack.tf/profiles/76561198297554259#!/compare/1513987200/1514073600

Sale 7:

Bulk for either keys for Time Warp Armored Authority

Seller: https://backpack.tf/profiles/76561198297554259#!/compare/1514937600/1515024000

Buyer: https://backpack.tf/profiles/76561198081129776#!/compare/1514937600/1515024000

Blizzardy Storm Ballcap:

https://backpack.tf/suggestion/5a76a1facf6c757d8305f868

Terror-Watt Harmburg:

https://backpack.tf/suggestion/5a76e5bf44325a0f2b1512d2

No current sellers.

Sales at:

22

24

27

29

22 (which without rounding, is 21.5) looks like an outlier, as it favours the trophy belt, and when used here, is pretty close to quickbuyers. Excluding 22.

Going with 24-29.