Sale 1:

Sold for Isotope Field-Tested Blitzkrieg Knife (24.5) + ~9.5 keys in pure/items

No outpost

Seller: https://backpack.tf/profiles/76561198096697494#!/compare/1535587200/1535673600

Buyer: https://backpack.tf/profiles/76561198453256831#!/compare/1535673600/1535760000

https://backpack.tf/item/5110356897

X = 34

Sale 2:

Sold for 31 keys + some metal

Seller: https://backpack.tf/profiles/76561198311498888#!/compare/1534723200/1534809600

Buyer: https://backpack.tf/profiles/76561198060937972#!/compare/1534723200/1534809600

https://backpack.tf/item/5110356897

X = 31

Sale 3:

Sold for 29 keys

Seller: https://backpack.tf/profiles/76561198060937972#!/compare/1535155200/1535241600

Buyer: https://backpack.tf/profiles/76561198121384478#!/compare/1535068800/1535155200

https://backpack.tf/item/5110356897

X = 29

Sale 4:

Sold with 3 keys for Burning Flames Platinum Pickelhaube

http://www.tf2outpost.com/trade/29761281 - B/O of 29

Seller: https://backpack.tf/profiles/76561198140316312#!/compare/1535760000/1535846400

Buyer: https://backpack.tf/profiles/76561198082511092#!/compare/1535673600/1535846400

https://backpack.tf/item/5110356897

Easily caps at the B/O

X = 29 (Capped by B/O)

Sale 5:

Sold for 27 keys

Seller: https://backpack.tf/profiles/76561198026040622#!/compare/1532995200/1533081600

Buyer: https://backpack.tf/profiles/76561198090757260#!/compare/1532995200/1533168000

https://backpack.tf/item/4960920846

X = 27

Other Sales:

https://backpack.tf/item/5110356897 (#1 & #2)

#1

Probably traded back and forth.

https://backpack.tf/profiles/76561198096697494#!/compare/1535500800/1535587200

https://backpack.tf/profiles/76561198267963372#!/compare/1535414400/1535500800

#2

Untraceable, probably 25 keys.

https://backpack.tf/profiles/76561198453256831#!/compare/1535760000/1536105600

https://backpack.tf/profiles/76561198140316312#!/compare/1535673600/1535760000

#3

Sold for Searing Plasma Bone Dome + 3 keys

No outpost

https://backpack.tf/profiles/76561198121384478#!/compare/1535241600/1535328000

https://backpack.tf/profiles/76561198096697494#!/compare/1535241600/1535328000

https://backpack.tf/item/5110356897

Can be used on the Bone Dome. Would most likely fit in the range here.

Sellers:

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

https://backpack.tf/item/5110356897 - 28 keys

https://backpack.tf/item/2587280175 - 40 keys

Sales at:

27

29 (2x)

31

34

Going with 27-34.