Sale #1:

https://backpack.tf/profiles/76561198192411802#!/compare/1544140800/1544227200

https://steamcommunity.com/market/listings/440/Unusual%20Bonk%20Leadwear

Sold on December 7, 2018 for $27.22.

$27.22 / $2.22 = ~12 keys

12 keys

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Sale #2:

https://marketplace.tf/items/30030;5;u61

Sold on November 28, 2018 for $31.

$31 / $1.71 = ~18 keys

18 keys

18 keys is clearly an outlier due to classifieds and other sales. Going with 12 keys.