Going with the past 2 months.

Sale #1:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305676157#!/compare/1552518000/1552604400

Bought for 15 keys and 12 ref.

15 keys

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Sale #2:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305676157#!/compare/1553382000/1553468400

Sold for 19 keys.

19 keys

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Sale #3:

https://backpack.tf/profiles/76561198021055079#!/compare/1548892800/1548979200

Sold for a Strange Festivized Specialized Killstreak Australium Flame Thrower (~18 keys) and 6 keys.

24 keys

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Sale #4:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198340634356#!/compare/1549666800/1549753200

Bought for 15 keys and 4 ref.

15 keys

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Sale #5:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305676157#!/compare/1551567600/1551654000

Bought for 15 keys and 12 ref.

15 keys

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Sale #6:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305676157#!/compare/1551999600/1552086000

Sold for 19 keys.

19 keys

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Sale #7:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198340634356#!/compare/1551135600/1551222000

Sold for 24 keys.

24 keys

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Sale #8:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198340634356#!/compare/1551826800/1551913200

Bought for 15 keys and 8 ref.

15 keys

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Sale #9:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198335791307#!/compare/1552172400/1552258800

Sold for 20 keys.

20 keys

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Sale #10:

https://bot.tf/stats/item?def=30570&q=5&ef=Spooky+Night&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198335791307#!/compare/1552604400/1552690800

Bought for 14 keys and 28 ref.

15 keys

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Sale #11:

https://marketplace.tf/items/30570;5;u3021

Sold for $30.13.

$30.13 / $1.80 = ~17 keys

17 keys

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Sale #12:

https://marketplace.tf/items/30570;5;u3021

Sold for $55.74.

$55.74 / $1.79 = ~31 keys

31 keys

Sales:

15, 15, 15, 15, 15, 17, 19, 19, 20, 24, 24, 31. Going with 15 - 20 keys.