Found another sale, but it didn't change the range. Oh well.

Sale #1:

https://backpack.tf/profiles/76561198055622215#!/compare/1547942400/1548028800

Sold with a for an Open Mind Vampire Vanquisher (18 keys)

18 keys

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Sale #2:

https://marketplace.tf/items/96;5;u60

Sold on December 26, 2018 for $28.

$28 / $1.79 = ~15.6

16 keys

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Sale #3:

https://backpack.tf/profiles/76561198846881416#!/compare/1547942400/1548028800

Sold with Muskelmannbraun (3.55 ref) and The Heavy Duty Rag (1.66 ref) for a Circling Peace Sign Bomb Beanie (15 keys), Graybanns (6.61 ref), Taunt: Rock, Paper, Scissors (20.5 ref), and Taunt: The Victory Lap (2.4 keys).

.15 + .46 + 2.4 + 15 - .04 - .08 = ~17.9 keys

18 keys

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Minis:

Open Mind Vampire Vanquisher:

https://backpack.tf/profiles/76561198243238038#!/compare/1545609600/1545696000

Sold with a Voltaic Hat Protector Universal Translator (15 keys) for a Sunbeams Puffy Polar Cap (33 keys).

18 keys

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Circling Peace Sign Bomb Beanie:

https://backpack.tf/profiles/76561198135268403#!/compare/1548460800/1548633600

Sold for 15 keys

15 keys