I found 3 reliable sales, i.e. exhibits 1, 2 and 4. Exhibit 3 is an outlier, see exhibit 5 where there are higher buy listing.

Exhibit 1: sale: 17 Apr, 2020:

Sold (to me) for 8 keys 50.33 ref (=> 9 keys)

https://i.gyazo.com/c3c8ca9cde93d51c48257f8fc4629422.png

Exhibit 2: sale: 04 Apr, 2020:

Sold for ~10 keys

https://backpack.tf/profiles/76561198146791360#!/compare/1585699200/1585958400

Exhibit 3: unreliable sale: 10 Apr, 2020:

Sold for ~ 7 keys

keys removed = 25 keys - (~16) keys for ks aussi knif - (~2) keys for Bills's

=> ~7 keys for this hat

Attempts to contact the buyer only resulted in him trying to fix the price.

https://backpack.tf/profiles/76561198109583337#!/compare/1586476800/1586563200

Exhibit 4: sale: 11 Apr, 2020:

Sold for 20 $ = 11 keys

I found the price by contacting the buyer directly and he was very helpful :)

https://backpack.tf/item/1109495871

https://marketplace.tf/browse/tf2?squality=5&seffect=33&sterm=Glengarry+Bonnet

https://backpack.tf/profiles/76561198089862063#!/compare/1583712000/1583884800

Exhibit 5: buy listing:

top buy listing at 8.4

https://i.gyazo.com/756ec68fd4153f508b715e0961923bf3.png