Sale 1:

Sold for Neutron Star Human Cannonball (26.5) + 2 keys

Seller: https://backpack.tf/profiles/76561198179707937#!/compare/1549238400/1549324800

Buyer: https://backpack.tf/profiles/76561198177615870#!/compare/1549238400/1549324800

https://backpack.tf/item/7178474310

X = 29

Sale 2:

Sold for Green Energy Sola Topi (Suggestion at 22.5)

Seller: https://backpack.tf/profiles/76561198113853563#!/compare/1549065600/1549152000

Buyer: https://backpack.tf/profiles/76561198117923240#!/compare/1549065600/1549152000

https://backpack.tf/item/5056595502

X = 23

Sale 3:

Sold for 19 keys

Seller: https://backpack.tf/profiles/76561198146666239#!/compare/1545350400/1545523200

Buyer: https://backpack.tf/profiles/76561198113853563#!/compare/1545350400/1545436800

https://backpack.tf/item/5056595502

X = 19

Marketplace Sales:

https://marketplace.tf/items/30549;5;u7

https://manic.tf/keyprice/?at=2018-12-16 - $52 / $1.65 = 31.52 keys

https://manic.tf/keyprice/?at=2019-02-06 - $52 / $1.81 = 28.73 keys

https://i.imgur.com/kf1lSAw.png - 29 key sale had a B/O of 30

X = 32

X = 29

Other Sales:

https://backpack.tf/item/5056595502 (#1)

https://backpack.tf/item/5049614941 - marketplace

https://backpack.tf/item/7017288601 - marketplace

#1

Untraceable

https://backpack.tf/profiles/76561198117923240#!/compare/1549065600/1549152000

https://backpack.tf/profiles/76561198337734785#!/compare/1549065600/1549152000

Green Energy Topi:

https://backpack.tf/suggestion/5c7b6add6780724c5f34f978

Sellers:

https://i.imgur.com/D869Emk.png

https://backpack.tf/item/5056595502 - 22 keys

https://backpack.tf/item/7178474310 - 27 keys

https://backpack.tf/item/5152395789 - 27 keys

https://backpack.tf/item/6858794698 - 30 keys

https://backpack.tf/item/5049614941 - 30 keys

Sales at:

19

23

29 (2x)

32

2 sellers at 27, and about a month unsold at 22. Excluding 29+.

Going with 19-23.

https://backpack.tf/profiles/76561198125791160#!/compare/1551484800/1551571200

https://backpack.tf/profiles/76561198337734785#!/compare/1551484800/1551571200

https://backpack.tf/item/5152395789

sold for 18 keys + 2 ref (though i'm guessing mods are already aware given 5 mod comments)

One of the sellers at 30 sold for Miami Nights Noble Nickel Amassment + 10 keys

https://backpack.tf/profiles/76561197964171516#!/compare/1552435200/1552521600

https://backpack.tf/profiles/76561198301654320#!/compare/1552435200/1552521600

https://backpack.tf/item/6858794698

Gonna be waaaaay too high, especially considering they resold to scrap in a day.

https://backpack.tf/profiles/76561198053717045#!/compare/1552521600/1552608000

https://backpack.tf/profiles/76561198337734785#!/compare/1552521600/1552608000

https://backpack.tf/item/6858794698

untraceable, most likely sold for ~18 keys, as that's what the buyer paid for the other one.

https://backpack.tf/profiles/76561198066822788#!/compare/1552348800/1552521600

https://backpack.tf/profiles/76561198131774728#!/compare/1544745600/1552521600

https://backpack.tf/item/5049614941

scanned/backpack transfer/whatever