Sale #1:

https://marketplace.tf/items/30807;5;u9

Sold for $50.

$50 / $1.81 = ~27.6 keys

28 keys

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Sale #2:

https://backpack.tf/profiles/76561198132182386#!/compare/1548374400/1548460800

Sold for a Cool Festivized Killstreak Pizza Polished Scattergun (Minimal Wear) (25.5 keys).

26 keys

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Sale #3:

https://backpack.tf/profiles/76561198132182386#!/compare/1547164800/1547251200

Sold for 27 keys.

27 keys

Sales:

26, 27, 28. Going with 26 - 28 keys.