Using past 2 months.

Sale #1:

https://bot.tf/stats/item?def=30920&q=5&ef=Holy+Grail&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198340634356#!/compare/1553900400/1553986800

Bought for 6 keys and 16 ref.

6.5 keys

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Sale #2:

https://backpack.tf/profiles/76561198198816727#!/compare/1552780800/1552867200

Untraceable.

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Sale #3:

https://backpack.tf/profiles/76561198045399485#!/compare/1552262400/1552348800

Untraceable.

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Sale #4:

https://bot.tf/stats/item?def=30920&q=5&ef=Holy+Grail&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198335791307#!/compare/1551135600/1551222000

Sold for 8 keys and 16 ref.

8.5 keys

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Sale #5:

https://backpack.tf/profiles/76561198097271466#!/compare/1553817600/1553904000

Untraceable.

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Sale #6:

https://bot.tf/stats/item?def=30920&q=5&ef=Holy+Grail&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198305676157#!/compare/1553468400/1553554800

Bought for 6 keys and 18 ref.

6.5 keys

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Sale #7:

https://bot.tf/stats/item?def=30920&q=5&ef=Holy+Grail&craft=1&aus=0&ks=0

https://backpack.tf/profiles/76561198340634356#!/compare/1553900400/1553986800

Bought for 6 keys and 16 ref.

6.5 keys

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Sale #8:

https://marketplace.tf/items/30920;5;u3003

Sold for $11.20.

$11.20 / $1.80 = ~6 keys

6 keys

Sales:

6, 6.5, 6.5, 6.5, 8.5. Going with 6 - 8.5 keys.